A heat alert has been issued for Napier as the mercury rises above 30C, while the top of the country is cloaked in humid air.

It comes after international and domestic flights were delayed this morning after fog settled over Auckland Airport.

Napier is currently the hottest place in the country, sitting at 31C. Gisborne closely follows on 30C.

Other extremes include Whangārei, Christchurch and Timaru on 27C.

MetService meteorologist John Law said Auckland temperatures were expected to soar to 29C today and not drop much lower as the day continued.

By 1pm the humidity in Auckland had reached 93%.

Law said temperatures were not expected to drop by much for the rest of the day, only falling to 21C overnight.

A heat alert has been issued for Napier, warning people to keep hydrated, seek shade and check in with vulnerable people, pets and livestock.

Law said the warm temperatures were the remains of a tropical system being dragged down from the north.

“It’s not as big as some of the systems we’ve seen, but it’s still bringing down that warmth and that moisture, leading to a really warm and humid start, which is also helping to form some of those misty, murky conditions around the coast as well.”

A scattering of showers is expected throughout today.

“Something to look out for,” Law said.

Law said the heat is only meant to hang around for today, before it runs more “seasonal” tomorrow, back down to the mid twenties for most places.

Meanwhile, a range of weather watches are in place for parts of the South Island.

A heavy rain watch is in place for the ranges of Westland south of Fox Glacier until 9pm today and Fiordland until 3pm.

The same watch will be in place for Marlborough south of Seddon and Canterbury north of the Rangitata River from 2am tomorrow morning until 10pm.

A strong wind watch has been issued for the Westland and Grey Districts and Fiordland from early tomorrow morning.

An airport spokesperson said six regional flights and two Australian flights, one heading to Sydney and another arriving from Melbourne, were delayed by the conditions.

The affected international flights were JQ202 to Sydney and JQ217 from Melbourne.

The fog restrictions were put in place at 5.50am and were lifted at 6.55am.