Auckland is about to be deluged by a tropical "atmospheric river" with rain to start saturating the city for 24 hours from dawn tomorrow.

Storm clouds are looming as the latest spell of bad weather fuelled from the tropics bears down on the country. Motorists in parched regions are being told to be wary of roads turning to skating rinks.

Brisk winds are beginning to sweep over the top half of the country and rain is poised to hit western regions of both islands this morning.

Western and northern districts of both islands are bracing for a tropical deluge in coming days, when up to 250mm of rain is expected in parts of the South Island and 150mm at the top of the country.

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Motorists driving on rain-slicked roads in areas that haven't seen rain for weeks are being told to watch for "summer ice" as dust and oil build-up combined with rain cause tyres to lose traction on the greasy surface.

This morning MetService added Auckland to its heavy rain warnings saying up to 120mm of rain would fall from 6am Thursday to 6am Friday with torrential downpours throughout.

Ferries to and from Auckland's Gulf Harbour are cancelled until further notice as conditions are too stormy for safe travel.

Heavy rain warnings have been issued for Northland, Auckland, Taranaki, Nelson, Marlborough Sounds and Westland as the front moves slowly across the country today and tomorrow. Watches are in place for Coromandel Peninsula, Waikato and Waitomo and Buller and northern Westland.

The heaviest rain is expected in the Westland ranges south of Otira, and Nelson from Motueka westwards and Mt Taranaki. Up to 250mm will fall in the coming 24 hours.

It's also likely to get extremely heavy at the top of the country, where up to 150mm is expected to drench Kaikohe north from this evening and Auckland starting during the morning rush hour.

MetService is warning the heavy rain may cause streams and rivers to rise rapidly.

Surface flooding and slips are also possible and driving conditions may be hazardous.

Other western regions will descend into dampness. Much of the North Island will get drenched as the wet weather moves east on Thursday and Friday.

Today Niwa said the low pressure swirling in the Tasman Sea over the next two days would direct a plume of moisture known as an atmospheric river from the tropics to the top of the North Island.

The rain is being welcomed by Horticulture New Zealand, which said drought conditions were threatening fruit and vegetable supplies.

"The dry conditions we have seen through early summer are putting fruit and vegetable growers under pressure to the point where some are having to make decisions about which plants and trees they may not be able to plant or harvest, and which may need to be left to die as scarce water supply is used to keep other plants alive," said Horticulture New Zealand chief executive Mike Chapman.

"No water means plants die and as a result, fresh fruit and vegetables are unavailable and prices go up because demand is higher than supply."

But relying on rain wasn't enough with the need for dams to ensure there was a reliable and adequate water supply.

MetService said although it was expected to be a wet week for many, eastern regions in the South Island remained sheltered by the Southern Alps. Canterbury was only expected to be sprinkled with a few mid-week showers at most.

The country would also be buffeted by strong northerly winds, particularly about coastal areas.

Motorists were asked to take care on the Desert Rd with strong winds buffeting the central volcanic plateau.

Today's forecast
Whangarei: Showers, strong northeasterly winds. 25C
Auckland: Showers, strong northeasterly winds. 20C
Hamilton: Cloudy, gusty northeasterly winds. 24C
Tauranga: Showers, northeasterly winds. 23C
Napier: Fine, gusty northeasterly winds. 27C
Wellington: Evening rain, northerly winds. 23C
Christchurch: Cloud, northeasterly winds. 26C
Dunedin: Light rain, northeasterly winds. 26C