In prehistoric times , the Waikato 's course has occasionally shifted to flow north through the Hinuera Gap into the Firth of Thames and from there into the Hauraki Gulf . The most recent occasion this is known to have occurred ended some 20,000 years ago , although it is possible that it also flowed north more recently , until about 1800 years ago . A remnant of this former course can be seen as a spur on Lake Karapiro to the south of the settlement of Piarere . The river 's current course is largely the result of the massive Hatepe ( Taupo ) volcanic eruption of 180 AD
Hatepe ( Taupo ) volcanic eruption
Hatepe eruption
Waikato River