Lake Rotoroa (Hamilton Lake) began forming about 20,000 years ago. Originally it was part of an ancient river system that was cut off by deposition material and became two small lakes divided by a narrow peninsula. With higher rainfall and drainage from the extensive peat land to the west, the water level rose so the narrow peninsula was drowned so forming one larger lake. To the north the lake is 8 m deep and in the southern (hospital) end 6 m deep. The old dividing peninsula, the start of which is still visible above water on the eastern side, is only 2 m below the surface.