Waikato is preparing for floods as heavy rain threatens to push already-swollen rivers past their limits.

Showers were expected to pound the region for 24 hours from 6am tomorrow, and, with rivers already high and catchments already saturated from last week's storm, Environment Waikato said it was preparing for the worst.

"The 70mm to 100mm rainfall predicted for parts of our region, if it eventuates, could result in higher Waikato and Waipa river levels than we saw last week, meaning potential for flooding beside waterways," emergency management officer Adam Munro said.

"We are urging the public and farmers to stay on the alert for information and be prepared to take action required to protect people, stock and property."

Mr Munro said Lake Taupo's level, which had been falling, would be closely monitored.

"That means we will have to continue working closely with Mighty River Power to return it to well below 375.25 metres above sea level, which is the top of its consented operating range."

Environment Waikato would also be liaising closely with emergency services and civil defence.

The MetService said Hutt Valley, Kapiti Coast, inland from Taranaki to Tongariro National Park, as well as parts of the South Island would also be hit by heavy rain.

"The wettest area is likely to be north of Otira to western Nelson through to Thursday morning with 170 to 220mm near the ranges through to early Thursday afternoon," MetService said.

Heavy rain in Westland was expected to ease today, but another burst was expected to hit the area tomorrow morning.

- NZPA