UPDATE 9.09AM: Rain is starting to fall over Northland and winds are starting to pick up as the first wave of dire weather hits the top of the country.

Much of the country is bracing for a rough 48 hours as a storm packed with the intensity of an ex-tropical cyclone sweeps across the North Island and top of the South Island.

Civil Defence is on high alert as the top of the North Island prepares for a pummelling from ferocious winds, torrential rain and enormous waves.

All of the North Island's west coast and the top half of the east coast is under threat of coastal flooding as the huge waves and strong onshore winds swell king tides.

At least one campground has been evacuated ahead of towering seas and tides threatening to flood coastal regions.

MetService meteorologist April Clark said wind gusts of up to 120 kilometres an hour could cause carnage.

"They will be quite strong, and that can mean that unsecured structures could be moved in that wind. People on the road on motorcycles could be buffered by that wind."

The storm system, the most powerful since September, is also expected to bring heavy rain to the South Island.

"We do have a heavy rain watch out for parts of Marlborough and parts of Nelson, so they'll be expecting those easterly winds, prolonged rain there, possibly heavy around the ranges and other areas," Clark said.

MetService forecaster Peter Little said the rain had already started falling in northern regions and wind was slowly picking up.

It was now 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 heaviest rain is expected in 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 winds of up to 120km/h are expected to hit Northland this afternoon before sweeping south.

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.

That is bad news for free-campers along the Bay of Plenty coast, who have been told not to ignore weather warnings and to move away from beaches.

Bay of Plenty council's duty flood manager Peter Blackwood said the sea is unpredictable and people shouldn't underestimate its power.

"There are a large number of campers from Waihi Beach right around to Cape Runaway past Waiho Bay, and they need to be really careful as a lot of people camp close to the sea, they need to be very weary of what the sea is doing.

"We can expect some quite large waves to come ashore. We are not expecting it to go into houses based on he current forecasts, but people need to be very vigilant, keep well clear of those waves, some of them could be quite large."