Heat alerts have been issued in four locations as MetService warns of continuing warm temperatures across the eastern coasts of New Zealand.

Gisborne, Napier and Hastings are all in for 32C days, while Blenheim is expected to record a 30C maximum.

"The temperature is forecast to be very hot for this location," messages on the MetService website read.

"We advise you to keep hydrated, seek shade, check in with vulnerable people and look after your pets and livestock."

Using 20 years of data, meteorologists have picked a threshold for 46 locations across the country, with the alert triggered if the mercury spikes.

Meteorologist Mmathapelo Makgabutlane told 1News it had been a warm start for eastern areas this morning and some places remained above 20C overnight.

"That would have been pretty nasty."

As of 11am, Kaikōura was the hottest place in the country at 29.8C with Gisborne not far behind on 29.6C.

In the major centres, Auckland and Hamilton were reaching 24C, Tauranga was hitting 27C, Wellington was around 22C, Christchurch was hitting 27C and Dunedin is around 20C.

Fire crews in Canterbury were working in the heat to quell a 3ha+ vegetation fire at Bridge Hill near Lake Pearson — which also closed SH73.

Southland was getting some relief from the warm temperatures, with a southwesterly wind cooling Invercargill and Gore to a manageable 16C and 19C respectively.

Makgabutlane said the sweltering start to summer was primarily due to two reasons.

One was the current westerly and northwesterly winds — known as "Foehn winds" — which could bump up temperatures, she said. Foehn was pronounced "fern".

As air ascended over topographical features such as the Southern Alps and North Island ranges, it cooled and — if there was enough moisture in the air — clouds could form.

A loss of moisture meant a hot, dry wind came down the other side. Drier air warmed faster because there was less water to heat.

"The second thing is we've had quite a lot of warm, moist air flowing over the country. Couple that with those westerly winds bringing those temperatures up, and we're seeing those hot temperatures as we've seen over the last few days," Makgabutlane said.

NIWA climate scientist Gregor Macara said it was not unusual to for temperatures to reach the low-30s in December in New Zealand.

"That said, the temperatures themselves are considerably higher than they would typically be at this time of year. Daily maximum temperatures have been about 10°C higher than normal for many places."

Climate change was "increasing the likelihood of such high temperature events in NZ" but was "not the primary reason for the current temperatures we are seeing", he added.