Christchurch and Dunedin can expect highs of 20C and 19C as temperatures in all South Island regions will be at least 4C above average, WeatherWatch said.
Auckland, large parts of the Waikato, Taranaki and the East Coast will also soar between 4C and 8C above average tomorrow.
It means Auckland residents can expect highs of 21C tomorrow and Saturday with Napier even tipped to hit a high of 24C, something WeatherWatch described as "exceptionally warm for the second week of May".
However, rain is expected to also make its way up the country as a "complex trough" arrives from the Tasman Sea and spreads across most of the country on Sunday.
The heaviest falls would be in western and central areas, but the upper North Island including Auckland would also see some rain on Sunday.
This front could also bring northerly gales, becoming severe from Auckland to Whanganui, and also Wellington, northern Marlborough and Nelson.
On Monday, a strong and showery southwest flow over New Zealand would ease, as a ridge developed - bringing more settled weather.
Tomorrow's weather
WhangÄrei
Rain developing morning, clearing evening. Northerly breezes. 22C high, 12C overnight.
Auckland
Partly cloudy, chance shower. Northeasterlies. 21C high, 15C overnight.
Hamilton
Morning rain then fine spells and chance shower. Westerlies dying out at night and fog developing. 21C high, 9C overnight.
Tauranga
Showers, clearing at night to fine spells. Northerlies dying out. 21C high,12C overnight.
Rotorua Showers, clearing at night to fine spells. Northerlies dying out. 18C high,9C overnight.
New Plymouth Early rain, then fine spells and chance shower. Northwest breezes. 19C high, 12C overnight.
Napier Light morning rain possible then becoming fine. Northerlies turning westerly in the evening. 24C high, 8C overnight.
Whanganui Early rain, then fine spells and chance shower. Northwesterlies. 20C high,12C overnight.
Wellington Rain easing early morning, then cloudy with a shower or two. Northerlies, strong at first. 18C high, 14C overnight.
Nelson Early rain, then fine. Northerlies, dying out evening. 19C high, 10C overnight.
Christchurch Fine. Light winds. 20C high, 7C overnight.
Dunedin Fine, some early morning and evening cloud. Light winds. 19C high, 10C overnight.
