An "atmospheric funnel" is bringing significant heavy rain and strong winds to much of the country today and into tomorrow.
Orange heavy rain warnings are in place for many areas, and wild weather is already hitting the tops of both main islands.
MetService said the severe weather was caused by a front moving eastwards, and was expected to last into Wednesday morning. It advised people to keep up to date with the latest warnings and watches.
Orange heavy rain warnings were in place for Auckland, Coromandel Peninsula, Bay of Plenty, Central North Island mountains, Taranaki Maunga, Tasman, Nelson, Marlborough, the Westland ranges, and at the headwaters of Canterbury and Otago lakes and rivers.
Some strong wind watches were also in place for parts of the North Island.
About 2000 homes and businesses across Northland are without power this morning after a night of heavy rain and strong winds.
In the Far North, lines company Top Energy said the biggest outages were in Kawakawa and Ćhaeawai, where 402 and 265 homes were affected, respectively.
In the Northpower area, more than 700 homes - most of them in the Kaipara town of Maungaturoto - were currently without power.
Weatherwatch's head forecaster Philip Duncan told First Up an "atmospheric funnel" had stretched right across the South Pacific.
"When you look at the rain map that covers the South Pacific, you see what looks like a funnel that goes from Vanuatu to Fiji to Tonga and it comes straight down in a v shape and points to the top of New Zealand and the top of the South Island.
"It's not necessarily the biggest rain band in the world, but the fact that it is being funnelled into one point or some narrow points means that we are going to get some very heavy rain stuck in some places like the top of the North Island, the top of the South Island and the West Coast."
Duncan said the top of both Islands were the biggest concern because they "don't normally get a month of rainfall in a day".
He said wind was also set to hit, and power cuts were likely in some places.
"It's pretty widespread but - again it's that tunnel shape - most of the worst of that wind is at the top of New Zealand.
"Really the worst of the winds look to be around Auckland, Northland, Coromandel Peninsula, and my hometown of Te Aroha in the eastern Waikato."
The worst of the weather was expected today, he said.
Tasman Mayor Tim King said the district was well-prepared, just over two weeks after the last flood caused widespread damage there.
Nelson Tasman Civil Defence said an emergency operations centre had been activated, and they were asking residents to travel only if necessary.Â
"With high tide at 12pm coinciding with the strongest winds, we are asking that people take care on all our roads (especially our coastal routes), and only travel if necessary."
Heavier rainfall in the region began just before 6am and would continue through until 6pm.
Up to 180mm is expected to fall across Tasman west of Motueka, and the heaviest downpours are expected between mid-morning and early afternoon.
Further north, the Waikato Regional Council is warning that forecast heavy rain and severe winds may cause landslips, surface flooding and bring down trees.
Coromandel in the east of the region is under heavy rain warnings and strong wind watches until tomorrow morning, and the rest of Waikato is under a heavy rain watch.
The regional council's flood co-ordinator Rick Liefting said landowners should move stock to higher ground, as the amount of rain could exceed land drainage schemes and flood paddocks.
He said the persistent rain of recent weeks had left river levels elevated and catchments were saturated throughout the region.
