Severe weather warnings have been issued for Northland, Auckland, Coromandel Peninsula, Bay of Plenty, Gisborne, Nelson and Marlborough.
Northland could expect 80mm to 100mm of rain by this afternoon, MetService said.
The rain should spread quickly southwards, totalling more than 100mm in Coromandel, Bay of Plenty and Gisborne by tomorrow morning.
Easterly gales will accompany the heavy rain, with gusts of 120km/h in Northland, Auckland and Coromandel Peninsula, and 130km/h west of the Kaimai Range.
An extra 26 firefighters and five fire engines were yesterday sent to Kerikeri as an emergency back-up to support local crews in the north before the storm hit.
Housing New Zealand also said it would make extra staff available over the weekend, with maintenance contractors ready to deal with any urgent repairs.
But residents in at-risk coastal areas have been told it's a case of "neighbour helping neighbour" to ensure safety.
Several of Auckland's eastern and northeast beaches have been warned of possible erosion and damage to beachfront properties from sea swells, particularly around high tide.
Boaties have been advised to secure their boats ahead of the encroaching storm and avoid going out to sea over the weekend.
Motorists have also been advised to take extra care on the roads, or stay indoors.
Several Auckland ferry services have been cancelled.
Elsewhere, electricity companies have warned customers to be prepared for the possibility of power cuts because of the wild weather.
They say people should stay clear of downed power lines or damaged electrical equipment and call their electricity provider immediately.
The storm should be clear of New Zealand by Monday, with only a few showers and brisk winds remaining in the south and east.
Canterbury is bracing itself for more flooding,. The Christchurch City Council says it is preparing for the worst case scenario following recent damaging floods in the area.
Around 22,000 filled sandbags will be ready for collection in various locations across the city by noon today, and all emergency response teams will be on standby.
Farmers are welcoming the prospect of rain soaking their parched land, particularly in the west coast of the North Island, Waikato, Auckland and Hauraki-Coromandel.
"I know there'll be a fair number of farmers in Waikato and Northland who'll dance a jig at the first drops of rain," said Federated Farmers adverse events spokeswoman, Katie Milne.
"Some on the North Island's west coast haven't seen decent rain since last November."
- Additional reporting Teuila Fuatai
