The Waikato Plains (the alternative name Waikato Basin is an ambiguous term as it can refer to the entire river catchment) form a large area of low-lying land in the northwest of the North Island of New Zealand. They are the alluvial plains of the Waikato River, the country's longest river with a length of 425 km (264 mi).

The plains can be divided roughly into the Middle Waikato Plain (also Middle Waikato Basin Hamilton Basin ), extending in all directions around the city of Hamilton, and the Lower Waikato Plain, nearer the river's mouth. The two are broken by the rough, low-lying hills of the Hakarimata Range, between Ngāruawāhia and Huntly, and the Taupiri Range.

The region is heavily populated by New Zealand standards, with many living in Hamilton towards the center of the plains.

The plains are an area of once swampy land, much of which was drained by the early settlers, such as the Morrin brothers in the late 18th century and is now intensively farmed. Dairy cattle, sheep, grain and maize are all farmed here, but it is dairy farming that is the staple of the local economy. The lower plain is also known for vineyards, and the middle plain has some of the southern hemisphere's top thoroughbred stables, notably around the towns of Cambridge and Matamata.

A considerable amount of the land is peaty, and significant sections especially in the north east are still undrained swamp. Dozens of shallow riverine lakes lie at the central and southern end of the lower plain, notably Lake Waikare.

This part of the Wailkato watershed is currently managed as the Middle Waikato Management Zone and the Lower Waikato Management Zone.

There is geological evidence that the Hamilton Basin portion was formed as a rift valley. The normal faulting associated with this may have become inactive before 350,000 years ago, although in lake sediments there is evidence of significant local earthquake activity as recently as 7600 years ago, but epicenters may relate to known current active fault systems to the north east of the plains.

The basement is Waipapa Morrinsville Terrane greywacke formed in the Late Jurassic to Early Cretaceous (160-120 Ma).

The Hamilton Basin originally formed under water so marine Pliocene beds underlie Quaternary deposits that date from about 1.8 million years ago. The oldest deposits are called the Puketoka Formation which contains clays, sands, breccias and ignimbrite sheets from pyroclastic flow reaching the basin. The younger Waerenga Gravels are composed of weathered greywacke debris presumably deposited in fans extending from the surrounding ranges. Even younger, and so closer to the surface usually, is the Karapiro Formation with rhyolitic sands and gravels which can be weathered to clay.

The Waikato River over, even the last 1800 years, has changed course many times in the basin. It has changed course even more spectacularly on at least four occasions in the last 100,000 years, flowing northeast from the region of the current Lake Karapiro, and exiting near Thames in the Hauraki Gulf leaving drainage of the basin to the Waipa River which has not always been a tributary to the Waikato River. Between 65,000 years to at most 25,000 years ago it drained the Waikato Plains but then drained through the Hauraki Plains for 6,000 years returning to drain the Waikato Plains from around 19,000 years ago. So only in recent times,again, has it flowed northwest to empty into the Tasman Sea near Port Waikato. This means the recent sedimentary deposits near the river are mainly altered volcanics washed down from the North Island Volcanic Plateau and broken down volcanic soils but there are layers of tephra/breccia from the many significant rhyolitic eruptions to the south where recent river flooding has not been disruptive. Layers of the fifteen layer Kauroa Ash Formation are found within both the Puketoka and Karapiro Formation depending upon where you are in the basin. The ash beds can be many meters thick, although thin out north of Hamilton, and include the eight layer 3 to 5 m (9.8 to 16.4 ft) thick Hamilton Ash Formation deposited between 350,000 and 100,000 years ago. There are also tephras derived from the andesitic stratovolcanoes of the Tongariro and Taranaki regions, as well as from Mayor Island/Tuhua. The airfall tephra layers of the last 50,000 years vary from between 0.5 to 1.5 m (1 ft 8 in to 4 ft 11 in) thick.

37°40′11.79″S 175°31′5.93″E / 37.6699417°S 175.5183139°E / -37.6699417; 175.5183139