The downpour blasting the North Island is beginning to ease as the heavy band of rain begins to scatter with the worst of it behind Auckland and Northland.

However, much of the east coast of the North Island wouldn't see a reprieve until late Monday.

Auckland and Northland were under a severe thunderstorm watch but the worst of the rain had passed over the areas between 12pm and 4pm on Sunday.

The band of rain had not fully passed through, but was becoming less widespread.

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"It's not looking likely that Northland will get it [heavy rain] again, it is looking less widespread, it looks like the worst is over", MetService meteorologist April Clark said.

Clark said it was still raining across the board but it was "certainly more scattered than before", though there were still chances of heavy rainfall.

Marlborough, which had been pelted by 198.5mm of rain in the past 24 hours in its Western Ranges, was beginning to come through the worst of it.

The region had not been subject to the downpours seen in the north but the precipitation had been persistent.

The Bay of Plenty, Coromandel Peninsula and Taupo would have to suffer through the heavy rain at least until Monday, Clark said.

"It is starting to break up a bit. It's starting to move to the east and break up, those regions will be in the heavy rain warning.

"That band of rain does stick around into Monday, the big thing is that the rain is becoming scattered."

Tuesday would be the same kind of deal, everything would ease off.

The West Coast of the South Island would be worse affected and the upper North Island could still get more rain, although not heavy.

MetService advised to refer to their weather warning watches as every area was going to be slightly different.

Earlier, Aucklanders were being warned to take care as an intense downpour set in across the city.

The National Institute of Water and Atmospheric Research said streams were rising fast across the city and shared an image of a Browns Bay stream nearing the top of its banks.

MetService said the heavy rain set in across Auckland about 11.30am, and had eased by 4pm.

A motorist told the Herald visibility on Auckland's Southern Motorway was atrocious around midday with traffic crawling at about 50km/h.

Inner city roads were "awash" with water, he said.

Heavy rain is turning roads into rivers, like Portland Road, Remuera. Photo / Connor Haythorne
Heavy rain is turning roads into rivers, like Portland Road, Remuera. Photo / Connor Haythorne

"It's absolutely bucketing down out there. Stay off the roads if you can. It's just horrendous."

The torrential downpour has forced the cancellation of today's Big Gay Out at Pt Chevalier's Coyle Park due to health and safety risks associated with electricity connections.

Niwa said there had been reports of surface flooding in Whangaparaoa, north of Auckland.

A severe thunderstorm watch is in place for most of the North Island and the top of the South Island.

The alert covered Northland, Auckland, Coromandel Peninsula, Waikato, Waitomo, Taumarunui, Taupo, Taranaki, Taihape, Whanganui, Manawatu, Tararua, Kapiti-Horowhenua, Nelson and Buller.

"A low pressure system over the Tasman Sea directs a moist northerly flow across New Zealand, and is forecast to move slowly eastwards during today and Monday. Expect widespread rain across the North Island and the upper South Island during this period," MetService said.

The heaviest rain is expected about Westland from Fox Glacier northwards, Buller, Nelson, Marlborough, western Taranaki, Tongariro National Park, Taupo, Rotorua, Bay of Plenty, Waikato from Morrinsville eastwards, Coromandel Peninsula and Great Barrier Island.

Rainfall of 160mm to 200mm is expected to accumulate about the Kaimai Ranges and Tongariro National Park tonight and tomorrow.

Parts of Waikato, Bay of Plenty, Rotorua, Taupo, Great Barrier Island and Coromandel Peninsula could expect 110mm to 160mm over the same period.

MetService said 20mm to 30mm could fall per hour.

Nelson could expect a further 100mm to 150mm on top of what has already fallen about the ranges west of Motueka, and 60 to 100mm elsewhere today and tonight.

Meanwhile, Models are increasingly suggesting New Zealand could be hit by the remnants of a tropical cyclone currently wreaking havoc in the Pacific.

But MetService has stressed things could change between now and a week's time, when more than half of current models project the system making landfall over the North Island.

Tropical Cyclone Gita, which has already caused considerable damage and flooding in Samoa, has strengthened to a severe category three storm and is set to further intensify as it makes its way across the Pacific.

It was tracking southwest-ward or westward over Tonga.

Climate scientist Dr Jim Salinger said two of the world's best forecasting models showed what would be an ex-tropical cyclone passing across New Zealand either February 19 or 20.