Farmers are moving stock off their land as the Piako River floods properties on the Hauraki Plains.

Several families were forced to evacuate their homes at Patetonga when the Piako River's stopbanks overflowed yesterday.

Federated Farmers Waikato president Chris Lewis says it's the worst flooding they've had in 50 years - and the flood prevention system is overrun.

He said "a lot of land" is under water and flooding will impact local farmers for months.

"It's not a good situation for those farmers, their staff and their family to be in."

He said the flood prevention systems are overrun because these kinds of water levels were never planned for.

"I'm sure lessons will be learnt out of everyone doing their best to make sure that the situation is properly managed."

The unusually wet weather in the past two weeks has seen the Waihou and Piako Rivers reach record heights.

Water which has spilled goes into ponding areas which are designed as part of the area's flood management scheme. High water levels in these areas are expected to last for the next few days.

Hauraki Civil Defence said a close eye is being kept on the stability and integrity of the area's stopbanks.