MetService New Zealand lead meteorologist Michael Martens said a tropical trough hovering over the North Island was behind the "incredibly humid conditions".

Martens said that mean fog, or 100 per cent humidity was likely this morning.

"That is unheard of. It is what you expect in the Cook Islands and the tropics."

A westerly change will bring dry air and a pleasant high for Thursday and Friday but the humidity returns as Tropical Cyclone Gita enters the Tasman Sea.

The category 4 cyclone passed just below Tonga overnight, hitting the nation with winds of 195km/h and 150-250mm of rain.

It is now moving towards the southern islands of Fiji and is likely to become a category 5 cyclone today.

Tropical Cyclone Gita is scheduled to pass south of New Caledonia on Saturday before travelling south into the Tasman Sea.

"Early next week there is a chance it may reach New Zealand, but the model is still uncertain," Martens said.

"Once it moves out of the tropics and the sea temperature decreases it will lose a lot of its power.

"Even if it gets up to a category 5 it will likely be a 3 or a mid-level 2 storm if it reaches here."

Heavy rain fell overnight around Northland, Auckland and through to Taupo and more is expected for the day for most areas north of Taupo, as well as potential thunderstorms.

The South Island is mostly fine today, except for a weak front hitting parts of the West Coast.

"Rain should ease there today. A few fronts will arrive there later in the week, though the East Coast should be relatively fine," Martens said.

Today's weather

Northland, Auckland, Waikato and Bay of Plenty:

Areas of rain, chance of heavy falls / thunderstorms otherwise fairly patchy. Humid north to northeasterly winds continue.

Highs: 25C-26C

Western North Island (including Central North Island):
Cloudy areas, may be a shower or two more so in the afternoon. Becoming drier again in the evening. West to northwesterly winds.
Highs: 23C-26C

Eastern North Island: Areas of rain, more so about Hawke's Bay where there could be heavy falls and even a thunderstorm or two. Gisborne and further south about the Wairarapa may be fairly dry with high cloud although a shower or two in the afternoon can't be discounted. Winds from the northerly quarter.
Highs: 25C-29C

Wellington:
Sunny spells with north to northwesterly winds, easing from afternoon.
High: 23C

Marlborough and Nelson:
Sunny, some cloud develops overnight. Afternoon northwesterlies for Marlborough, afternoon northerlies for Nelson.
Highs: 26C-30C

Canterbury:
Sunny weather with afternoon east to northeasterly winds, some cloud moves in later in the evening or overnight.
Highs: 24C-27C

West Coast:
Morning cloud with a drizzle patch or two then sunny areas increase from afternoon, light winds tend southwest after midday.
Highs: 22C-27C

Southland and Otago:
A few showers pass through Southland late morning then into Otago (especially in the east) late afternoon / evening. Showers about eastern Otago may be heavy then clearing at night. Before showers move in conditions are mostly sunny with northwesterlies, changing south to southwest as showers move in.
Highs: 18C-27C