The Organ Pipes National Park, abbreviated as OPNP, is a national park located in the Central region of Victoria, Australia. The 121-hectare (300-acre) protected area was established with the focus on conservation of the native flora and fauna, and preservation of the geological features in the Jacksons Creek, a part of the Maribyrnong valley, north-west of Melbourne. It is situated in a deep gorge in the grassy, basalt Keilor Plains.

Within Organ Pipes National Park, the valley walls of Jacksons Creek expose Pleistocene volcanic rocks of the New Volcanic Group. These 2.5 to 2.8 million year-old basalt lavas, commonly known as trap rock, fractured during cooling into vertically standing, hexagonal basalt columns. These columns are locally known as the "organ pipes" for which this park is named. Over the last one to two million years, the slow cutting by Jackson Creek of its valley down into the basaltic plains and through the underlying trap rock exposed these geological structures. The bottom of the valley of Jackson Creek also exposes a prehistoric buried creek valley, which is cut into 400 million year-old (Silurian) mudstones and sandstones. The bottom of this buried valley contains ancient creek gravel. Both the ancient river valley and the Silurian sedimentary rock lies buried beneath the basaltic volcanic rocks of the New Volcanic Group. Marine fossils found in the Silurian sedimentary rocks demonstrate that they accumulated beneath a prehistoric ocean.

A Friends' group, (the first in Australia) the "Friends of Organ Pipes" (FOOPS), comprising conservation activists to support the efforts of rehabilitation of the OPNP's indigenous flora and fauna, supplemented the work of the Victoria Park system under which the OPNP was declared a National Park. The park's importance to the whole region as a "center for education about the geology, flora and fauna of the Keilor Plains, and the restoration of degraded land" is important. With its inclusion in the IUCN Category III (Natural Monuments) of the United Nations' list of National Parks and Protected Areas, there is a greater recognition of the need to protect or preserve outstanding natural features.

The national park, designated to cover an area of 121 hectares (300 acres), is situated 20 kilometres (12 mi) to the northwest of Melbourne city. It is part of the Victoria Park system. It is approachable from the Calder Highway. The road network around the park consists of the Calder Highway on the south and the Calder Raceway also on the southern direction but close to its entrance. The Jacksons Creek, which is at the heart of the park, is bounded by the City of Hume on the northern side while the City of Brimbank is to its south; Deep Creek and Keilor Plains to the west of Melbourne are also part of this geographical setting. The complementary areas to the park are the Holden Flora Reserve (65 hectares (160 acres)), Holden School Site (25 hectares (62 acres)), Derrimut Grassland Reserve (154 hectares (380 acres)) and Laverton and North Grassland Reserve (52 hectares (130 acres)). Out of the total area of 121 hectares (300 acres), the conservation area is 107.3 hectares (265 acres) (88.6%), conservation and recreation area is 13 hectares (32 acres) (10.8%), visitor center accounts for 0.7 hectares (1.7 acres) (0.6%), and 2 hectares (4.9 acres) (1.6%) is the special management area. The park is drained by Jacksons Creek. This with Deep Creek and Mount Emu creek are the major tributaries of the Maribyrnong River, which has a catchment area of 1,300 square kilometres (500 sq mi). Jackson Creek rises in the Wombat State Forest, flows east to the Roslynn Reservoir, joining Deep Creek near Sydenham Park. It outflows from the reservoir through Gisborne, Sunbury before flowing through Organ Pipes National Park.

The park lies in a rain shadow area. According to records maintained at Melbourne Airport since 1972, the average annual rainfall in the park area is 580 millimetres (23 in) (occurs during winter and spring). Occasionally, heavy rainfall occurs in the area and one such event caused heavy floods during winter in the flood plains of Maribyrnong, in September 1993, causing loss of vegetation, destroying the viewing platform and also scouring the banks of the creek.

The earliest settlers in the plains of the park area and its vicinity were the Australian Aboriginal Woiworung people of the Kulin nation in the Yarra and Maribyrnong River valleys. Jackson Creek formed part of the boundary between two clan estates, Marin–Bulluk and Wurundjeri–Willam. They were farmers and hunters who lived on the forest products and harvested grass species and spear grass (Stipa spp.) that were grown and regrown in the area of the Keilor Plains after burning every year. The climate and rainfall also favoured growth of native grasses in the rolling plains. Animal species such as kangaroos, dingoes, tiger, bandicoots, gliders and platypuses were common. Bird species of cockatoos, kookaburras, quails, finches and hawks also flourished. The vegetation was lush with grass lands and plenty of plants with flowering blooms. Their habitation of the area is established by archaeological findings in the area in the form of their campsites and isolated artefacts within the park. The environment along Jackson Creek environment facilitated shelter, water and plentiful food for their original settlement.

In early 19th century, Europeans from Tasmania started settling in the area. They were chiefly responsible for introducing several species of exotic plants and animals into the area as they found the terrain to be drab and filled with strange animals like kangaroo. They introduced new variety of plants such as boxthorn hedges for fences and trees such as oaks, willows and pine, creating an ambiance of their homeland. Kangaroos were hunted, rabbits and furred animals were killed for their valuable skin. This had ill effects on the native vegetation and faunal species. So much so that "high artichoke thistles blanketed the creek flats and slopes, horehound had spread everywhere, boxthorn bushes crowded the slopes and plains, and other weed species filled the gaps. Erosion gullies scarred the steep slopes. Rubbish was piled here and there".

During this period, there was a hamlet near the Organ Pipes and a suspension bridge across Jacksons Creek to reach a school and farm lands that surrounded the village. The Hall family lived here between 1870 and the 1920s; their plum tree orchards still exist. The European settlers' period is also borne out by archaeological artefacts in the form of bluestone walls. The stables in the park area were built by the Halls in which the Hayes family lived, and a Jackson Bay fig tree is located near the Information Center. The Bartlett family lived here from 1934 to 1948 and the last owner was E. A. Green. His estate became the property of Stan Payne which was eventually donated to the Organ Pipes National Park in 1972.

Consequent to the deterioration of the environmental conditions, the well-meaning, environmentally conscious citizens decided to rehabilitate the environment by reinvigorating the area. The first step towards this end was initiated by the National Park Service in 1972 by declaring initially an area of 65 hectares (160 acres) of the land donated by the E. A. Green Charitable Foundation as a national park. Under Schedule 2 of The National Parks Act 1971 (Vic.), the area was declared as National Park on 12 March 1972. There were several objectives of this declaration, but the intent was to restore the park with both direct and indirect interventions to improve the ecosystem. Direct intervention came in the form of planting indigenous species as seedlings, and indirect format was to de-weed the area. As a result, the overstorey plants of indigenous variety and understorey species are now well established.

An area of 20 hectares (49 acres) of land was added under the National Parks (Amendment) Act 1978 (Vic.) and another 36 hectares (89 acres) of Crown land (including land acquired previously forcibly for the Park) was added on 4 June 1997. The National Parks Act enjoins on the Director of the park "to preserve and protect the natural environment of the Park, and its natural and other features." It also stipulates the use of the park by the public for enjoyment, recreation and education, apart from the needed research. The Park Regulations 1992 Section 4 (Objects) is related to visitor activities and Section 17 of the National Parks Act governs the management of the Park. The Park is also reserved as a "Geological Monument" under the final recommendations for the Melbourne Study Area (LCC 1977) and also continues as a National Park for the Melbourne Area District 1 Review (LCC 1987). Fire Protection Plan, Port Phillip Area (former Melbourne Region) (CNR 1996); Draft Conservation Program for Native Grasslands and Grassy Woodlands in Victoria (CNR 1992) and Code of Practice for Fire Management on Public Land (CNR 1995) are some of the other legalese connected with the park.

The particular site was chosen because of the presence of several interesting geological features, including the "Organ Pipes", themselves a pattern of vertical pipe-like rock structures exposed by the erosion of the creek, and a tessellated pavement, where the rock surface has been split into regular "tiles" by joints related to columnar jointing.

The Keilor Plains, where the geomorphological and geological features of heritage value are located, is part of the greater Western Volcanic Plains which cover a large part of western Victoria and extend into south-eastern South Australia and are stated to be the third largest lava plains in the world. These landforms were created by the incision of the Jacksons Creek into thick lava flows that erupted about 2.5 to 2.8 million years ago during the Pleistocene Period. These lavas overlie steeply dipping Silurian sedimentary rocks.

Sedimentary rock and fossils found in the park are also dated back 400 million years, an indication that the area was once a sea. The rocks seen in the park have mostly dark grey or brown colour. Graptolites are seen as fossils of sea snails, sea worms and extinct floating animals in the rocks here. About 2.5 to 2.8 million years ago giant lava flows known as a flood basalt spread across the land from surrounding volcanoes.

Rock exposures of Silurian mudstones and jointed structural features of basalt are spread at many locations in the form of Basalt cliffs, boulder screes and sedimentary escarpments. The geological features in the Park, all of volcanic origin, have been categorized under the broad heads namely, the Organ Pipes, Tessellated Pavement, Rosette Rock, Scoria cone and the Sandstone layer.

Jackson Creek, which meets with Deep Creek to become the Maribyrnong River, has slowly worn a deep valley in the basaltic plain formation of hard, dark rock revealing the old volcanic geological formations such as the hexagonal basalt columns known as the "Organ Pipes"; the lava deposit in the bed of the creek is of the order of 70 metres (230 ft). As the lava cooled over several years the interior molten lava got insulated and developed into undisturbed columns of basalt (with uniform composition) as the lava heat dissipated. The cooling effect caused shrinkage and as scientifically elaborated:

"This shrinkage caused tension in the rock mass. Vertical (upward and downward) tension could be accommodated by the elastic molten rock beneath but horizontal tension could not be relieved and so the basalt cracked. The rock usually cracks in a hexagonal pattern (six sides), but columns with up to eight sides are found. The rock was still hot (about 400 °C (752 °F)) when the columns were formed. Further contraction took place as the rock lost its remaining heat; this was relieved by horizontal cracking, causing some columns to look like stacks of Dutch cheese."

As seen now, the rock formation rises to a height of 20 metres (66 ft) and is considered as the best example of columnar jointing in Victoria.

The unusual shape of Jackson Creek Organ Pipes is attributed to the columnar structure which is rare and occurred at strategic location where the down cutting action of the stream created the post-volcanic valley. The Organ Pipes is so named as it resembles a pipe organ. The width of each pipe in the formation is about 1 metre (3 ft 3 in); only some of the columns are vertical while most other smaller columns around the Pipes are either inclined or horizontal.

Tessellated pavement is an assortment of basalt columns that evolved as a result of erosion by the Jackson Creek, which is seen in the form of pavements about 250 meters upstream of the Rosette Rock. Frequent access to this formation is likely to cause deterioration of this formation.

Rosette rock is a radial array of basalt columns. Rosette rock formation seen about 500 metres (1,600 ft) upstream of the Organ Pipes is an overhanging rock on the northern bank of the stream. It is also an outcrop of basalt but with a radial array of columns akin to the spokes of a giant wheel. Its formation is attributed to the cooling of "a pocket of lava, probably in a spherical cave formed from an earlier lava flow".

The sandstone layer seen in the Jackson Creek has been dated to about 400 million years old. They are made of quartz and quartzite gravel, and are located midway between the car park and the Organ Pipes. They are stated to be part of a deep lead—an ancient stream bed buried by a lava flow, and later revealed by the downcutting of Jackson Creek. Deep leads were mined for gold in Ballarat during the Gold Rushes.

The volcanic soils in the park formed above basaltic rock base are found in the valleys and have gentle or flat slopes. Soil types have been classified as zonal types comprising red-brown earths (dominant type in the park with low permeability), grey duplex soils in flat areas (with high permeability) and uniform medium loams (on steep slopes on the southern direction of shallow depth and well drained); azonal soil types consisting of alluvial soils (fertile soils in the park consisting of clay loams to gravels suitable to grow tolerant plant species like river red gum and river bottle-brush), colluvial soils (at the toe of steep slopes of dark grey to dark brown in colour, suitable to grow wide variety of vegetation) and lithosols (reddish brown soils on steep slopes and frequently support boxthorn and indigenous lightwoods (Acacia implexa); and other lithosols which are pre-basaltic sands and gravels seen near the foot of Grey Box Gully and in lower slopes of the Jacksons Creek valley.

Soils in the park have been subject to erosion, weed growth, human activity and animal burrowing, particularly by rabbits. They are generally found not suitable for plant growth due to their poor water holding property. The degradation of the soils has been arrested with extensive re vegetation.