Electricity generator Mighty River Power has been urged to spill water from its dam at Lake Karapiro to help the upper Waikato River hydro system store for rain from a big storm expected to hit on Friday.
The Waikato regional council called for the spill today after the Metservice warned more than 100mm of rain could fall on the Taupo catchment on Friday.
"There is a possibility in the higher areas that they could get up to 100mm, with lesser amounts at lower altitudes - significant enough that I imagine it will concern them, given that Taupo is so full," said Metservice forecaster Andy Downs.
The Metservice is expected to put out severe weather warnings tomorrow for the likely effects from a low pressure system forecast to cross southern New Zealand late tomorrow and Friday.
Tonight its website showed heavy rain in the hills and ranges in the western North Island and down the West Coast of the South Island.
The "active" front will not only bring heavy rain but possibly also northwest gales over northern and central New Zealand.
Heavy rain is likely in the ranges of Westland, Buller, Northwest Nelson, the Tararua Ranges, the hilly country from Mt Taranaki to the Central Plateau late Thursday and Friday, and the eastern Bay of Plenty, especially near the ranges, will be hit on Friday and early Saturday.
A southerly flow of cold air spreading into the deep south on Friday or Saturday also meant there was likely to be heavy snow down to low levels in inland Southland and South Otago.
The heaviest falls would be around the main divide, and people trying to drive through passes across the Southern Alps might find there was a lot of snow, said Mr Downs.
The regional council's hazards manager, Adam Munro, said the call was a precautionary measure to help ensure that the lakes upstream of Karapiro didn't come under significant pressure if the forecast rain eventuated on Friday.
He said the effects of the extra flows from Karapiro on low lying areas of Hamilton would be closely monitored, and the volumes being released from Karapiro might be increased tomorrow.
The releases from Karapiro, and the rain that was due to fall over Taupo on Friday, meant Waikato River levels were likely to stay higher for longer, and it would potentially take more time for land affected by the recent flooding to drain," he said.
