Waikato Regional Council says the spilling of water from the Karapiro Dam is a precautionary measure to allow for more rain.
Further rain is predicted in the Taupo catchment, to flow into an already swollen Waikato River.
Adam Munro from the council says it's working closely with Mighty River Power to ensure adequate storage in the hydro lakes along the river in case of extra rain.
"We are not expecting to create any great issues down steam. We are allowing water to pass trhough without any great concerns," he says.
Mr Munro says MetService is predicting 100mm of rain in the catchment on Friday.
He says while this wouldn't normally be a problem, land in the region is already saturated from recent rain.
Meanwhile, Civil Defence in Hamilton says it's keeping a close eye on the level of the river near two low-lying streets but it isn't expecting problems.
Paul Blewman from Civil Defence says at just over 16m the river would reach the road kerb and some surface road flooding is possible.
The regional council is not predicting the peak to reach 16m.
