Lake Rotopounamu (also known as Greenstone Lake, from the Maori (lake) and (greenstone)) is a secluded lake in the Pihanga Scenic Reserve, in the Tongariro National Park in New Zealand's Central North Island. It lies at the northwest foot of Mt Pihanga, and was formed by a landslide around 10,000 years ago. The lake is fed by seven streams, but has no visible surface outflow. It is believed to drain underground.