Weather: Sub-zero temperatures, eight-metre waves as southwesterly blows through

New Zealanders are set to shiver through the start of the week as a southwesterly front sweeps through the country, bringing sub-zero temperatures and strong winds.

The weather was "frigid" on Sunday, according to MetService, with much of Otago and Southland failing to hit 10C.

Overnight, the temperatures were set to "plummet". Twizel was picked as the coldest place, with a low of -4C, followed by Christchurch and Timaru with -3C.

The chill is expected to hang around for a few days, with frost in parts of the south, before temperatures improve in the latter part of the week.

Meanwhile, boaties and surfers were being advised to stay on dry land as huge waves pummel parts of the coast.

A heavy swell warning was in place around Wairarapa, from Turakirae Head to Mataikona, until 6pm Monday.

MetService said the waves were expected to surge to between 5.5 and six metres on Sunday afternoon, easing to four metres by Monday evening.

Further around the coast, another heavy swell warning was in place for Wellington from Cape Terawhiti to Turakirae Head until 3pm Monday, with waves expected to top out at 4.5 metres.

On Sunday, "strong, sustained winds" saw the swell hit seven or eight metres off some South Island coasts, according to a MetService update.

Marine warnings were in force for nearly all of the South Island and parts of the North over the coming days.

MetService said the "disturbed" southwesterly flow was set to impact Aotearoa until Thursday, with the wind gusting up to 40 knots in places - about 74km/h.

There were gale warnings for Castlepoint, Chalmers, Chatham Islands, Conway, Foveaux, Grey, Milford, Rangitata, Stephens and Bream Head to Cape Colville.

In some of those areas there would be "very rough seas", MetService said.

A strong wind advisory had also been issued for Hauraki Gulf, Waitematā Harbour, Manukau Harbour, Coromandel and Bay of Plenty, with gusts of up to 35 knots (65 km/h) expected.