Lake Wainamu is a dune lake that formed during the last 6,500 years when the Wainamu Stream Valley was dammed by dunes of sand blown inland. A decline in water quality was noted in 1990s and was thought to be caused by exotic fish accelerating the loss of the submerged plant life. In response the Auckland Regional Council removed over 9,000 exotic fish (particularly perch, goldfish and rudd) between 2004 and 2007. In 2009, grass carp were introduced to the lake in an attempt to rid it of introduced egeria.