Beach communities are being told to prepare to move to higher ground and have supplies ready to evacuate.
At least one campground has been evacuated ahead of towering seas and tides threatening to flood coastal regions.
Those holidaying at the Piha Domain Camping Ground were urged to evacuate ahead of the enormous swells and super-charged tides saying conditions could put lives at risk.
Many others in campgrounds across the top of the North Island are preparing to weather the storm - fastening tents, collapsing awnings and removing anything that risked flying away in hurricane-strength winds.
MetService forecaster Peter Little said the rain had already started falling in northern regions and wind was slowly picking up.
It was blowing 70km/h at Cape Reinga.
A swathe of weather warnings have been issued.
Heavy rain and destructive gales are set to bring havoc for the next two days.
The worst rain will bring deluges to Northland, Auckland, Coromandel Peninsula, Bay of Plenty, Mt Taranaki, Marlborough and Nelson.
MetService is forecasting up to 160mm of rain to fall over Mt Taranaki and the Nelson and Bay of Plenty ranges.
Coromandel is expecting 150mm and Northland 120mm, and Auckland is in line for up to 90mm beginning mid-morning, followed by torrential downpours this evening.
Damaging hurricane-strength gales of up to 120km/h are due to hit the top of the island at 5pm and towering waves measuring more than 6m are expected to arrive in the Far North an hour later.
Strong onshore winds mean that king tides today and Friday, coupled with waves the size of a double-decker bus, pose a threat to many low-lying communities.
All northward-facing bays in Auckland, coastlines from Northland to the Bay of Plenty and the Firth of Thames, and the North Island's west coast from tomorrow, are at risk.
Waikato Civil Defence has already set up the Emergency Operations Centre in Coromandel, a region swollen by tens of thousands of holidaymakers.
Campers in coastal areas and flood plains were yesterday told to move to higher ground amid fears flooding would leave them cut off.
Niwa warned the approaching storm bore similarities to ex-Tropical Cyclone Ita, which struck at Easter three years ago. It inundated coastal communities across the upper North Island and blew buildings to pieces on the South Island's West Coast.
The storm will move down the North Island today before hitting northern and eastern regions of the South Island on Friday and clearing the country Saturday.
