A southern storm is brewing below New Zealand and bringing heavy rain and potentially damaging winds and 8m waves onshore.

The North Island should escape most of the action however, as a ridge of high pressure is ensuring fine conditions and temperatures in the late teens to early 20s until Saturday.

MetService duty forecaster Mickey Malivuk said temperatures may even increase further later in the week as a band of warm subtropical air pushes over the North Island.

Auckland is in for a fine Tuesday and a high of 19C. Similar conditions are forecast through Northland, Waikato and Taranaki.

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Tauranga will crack 21C today, and so too will parts of the East Coast, where Gisborne will reach 20C and Napier 21C.

Most of the land-based weather action today will be at the bottom of the South Island, where gale winds and heavy rain roll in today and tomorrow.

Severe gale wind warnings are in force for Fiordland south of Doubtful Sound, Southland and Clutha from 10am to 3am tomorrow, and Otago excluding Clutha, and Canterbury south of the Rakaia River from 2pm to 3am tomorrow.

Northwest wind gusts may reach 120km/h in exposed areas. Dunedin may get wind gusts up to 90km/h this afternoon.

A strong wind watch is also in place for Wairarapa and Hawkes Bay south of Hastings until 6am tomorrow, and the Canterbury High Country.

There is a moderate chance of warning amounts of rain in northwest Nelson on Saturday, while the risk is low for the remainder of Nelson, Buller, Westland and also for northern Marlborough.

The winds are part of a "rare" swell brewing just below New Zealand.

At the moment waves in the open ocean are up to 15m, Malivuk said.

Swells are rising to 8m today about the Fiordland and Southland coasts.

"Those areas are used to big swells, but this is quite a rare event to have 8m swells hitting the coast. It also has a really long period, so it will be quite damaging."

The West Coast is in for a wet Tuesday, and the east coast will stay dry, but windy.

Tomorrow the front weakens and moves away from the South Island, and the ridge over the North Island spreads south.

It will sit over the whole country until Friday, before a low developing in the Tasman moves on to the West Coast.

"It is looking like quite a major low," Malivuk said.

"At the moment it is dragging warm air over the North Island, but as it moves closer there is potential for heavy rain."

The low will start out in the South Island Friday and Saturday, before moving over the North Island Sunday.

More than 100mm could fall over a 24-hour period on the West Coast Friday and Saturday, and on Sunday in the North Island as the low shifts northwards.

Today's weather

Whangarei

Fine. Southwesterly breezes. 20C high, 10C overnight.

Auckland A fine day. Southwesterly breezes. 19C high, 13C overnight.

Hamilton Partly cloudy. Southwesterly breezes. 19C high, 10C overnight.

Tauranga Fine. Southwesterly breezes. 21C high, 12C overnight.

New Plymouth Partly cloudy. Chance shower from afternoon. Westerlies. 18C high, 13C overnight.

Napier Fine. Northwest developing afternoon. 21C high, 10C overnight.

Wellington​ A fine day. Northwesterlies strengthening. 18C high, 14C overnight.

Nelson​ Fine, some evening high cloud. Northerly breezes. 18C high, 10C overnight.

Christchurch​ Fine, high cloud at times. Northwesterlies, becoming strong and gusty. 19C high, 6C overnight.

Dunedin​ Cloud increasing, a period of evening rain. Northwesterlies, strengthening to gale gusting 90 km​/​h. 19C high, 10C overnight.