Searing heat today will give way to powerful ex-cyclone later this week with 'a bit of coastal inundation' possible, forecasters say

The hot weather is expected to reach its peak today, but MetService is warning that the latter part of the week could bring powerful winds and very heavy rain.

Central Otago and Canterbury will continue to see the highest temperatures today, with the national high predicted for Alexandra - about 37C.

Meanwhile, rainfall is predicted to arrive in the North Island from Thursday.

Cyclone Fehi, which is currently northwest of New Zealand, should lose its cyclone status by about midday today, but meteorologist Tom Adams said it is still a threat because "when it comes near New Zealand it will intensify greatly".

"It still won't be a cyclone again but it will be a powerful storm regardless," Mr Adams said.

He said strong winds would pummel "pretty much the whole country" with a good chance of them being severe in Taranaki, Wellington, the West Coast and Nelson.

"There could be a bit of coastal inundation," Mr Adams said.

Along with wind, heavy rain is set to fall on much of the South Island and large parts of the North Island.

Meanwhile, fine conditions are expected throughout most of the South Island today, apart from morning cloud around Hokitika and Greymouth, and Milford Sound will see showers kick in this afternoon.

There will be some very hot temperatures along the East Coast too, with Ashburton hitting a high of 33C and Blenheim reaching 34C.

A little bit of cloud cover will form first up over the North Island's east this morning, but it will pass quickly.

A perfect day is set for the Bay of Plenty today once the overcast morning clears, but Rotorua, Whakatane and Tauranga could see some light rain first thing this morning.

A fine day is due in the Upper North also, with clear skies and warm temperatures, including a high of 27C in Auckland today.

There is, however, the chance of some showers in Northland.

Tomorrow it will be mainly fine across most of the country, although there's heavy falls heading for the South island's west, spreading to the south by the evening.