Lake Kainui is a peat lake of about 25 hectares in the Horsham Downs area of Waikato, east of Ngaruawahia and north of Hamilton. It is one of a series of lakes in the area, and is also known as Lake D or Lake Rotoaira. Lakes A, B and C are nearby. A walkway around the lake was opened in 2018, and public access to the lake itself and Lake Kainui Reserve was arranged by the Waikato District Council. A short access road off Lake Road leads to a parking area and the reserve. There is also a boat ramp and a toilet.

Various activities are permitted in the reserve and on the lake, but some activities require a permit and access to the boat ramp requires a key. The reserve may be closed to the public in May, during the duck shooting season. The walkway around the perimeter of the lake is about 3.1 km long, mostly well formed with a gravel surface, although there are also areas of grass. There are no steps and only the gentlest of gradients, with an elevation difference of only a metre or two along the entire track.