Auckland and Hamilton were expected to climb higher than their shared record of 17.8C and New Plymouth and Nelson are expected to beat their previous top temperatures of 16.4C and 15.5C.
The South Island was also poised to be warmer than usual. Invercargill, Dunedin and Christchurch are expected to come close to their record temperatures.
MetService forecaster Kyle Lee said the high temperatures were because of the encroaching system about to unleash its rainmaking furore on New Zealand later today.
Rain is expected to start on Great Barrier Island, Coromandel Peninsula and the Bay of Plenty around midday tomorrow and last until Thursday morning.
Up to 180mm of rain could fall over the two days.
Mt Taranaki and Tongariro National Park could also expect heavy rain tomorrow.
Meanwhile Niwa scientists were gearing up for a mid-winter dip and with a balmy 15C forecast they were encouraged to leave wetsuits at home.
The winter solstice arrived at 10.34am today, when the sun's position reached its farthest point north of the Equator. Today brings the least daylight of the year.
In Auckland, this equates to nine hours, 37 minutes and 58 seconds, and in Invercargill, eight hours, 35 minutes and 5 seconds worth of daylight.
During the winter solstice there was no sunlight at all south of the Antarctic Circle; conversely, it was the summer solstice in the Northern Hemisphere, when the sun was visible day and night in areas just south of the Arctic Circle.
Many Northern Hemisphere countries consider the summer solstice the official start of summer.
In New Zealand, Australia and South Africa, winter begins on June 1 and ends on August 31, otherwise known as the meteorological winter, combining the months with most similar weather together.
After today, daylight would begin to increase.
Most cities in New Zealand would have two or three minutes more daylight by the end of the month.
By the end of July, the additional daylight would reach nearly 38 minutes in Auckland, 45 minutes in Wellington and 54 minutes in Invercargill.
The solstice has been celebrated early at Stonehenge Aotearoa and the Antarctica.
Around 40 people gathered on Sunday to watch the sun set over the heel stone at the replica Wairarapa monument.
Unfortunately cloudy conditions obscured the sun, said Stonehenge director Kay Leather.
Those working at Scott Base have marked the halfway point with an annual mid-winter meal.
Forecast
Whangarei: Cloud increasing, odd shower. H 19 L 16
Auckland: Cloud increasing, shower possible. H 18 L 15
Hamilton: Mostly cloudy. Light rain at night. H 17 L 12
Tauranga: Mainly fine, evening cloud. Light winds. H 18 L 14
Napier: Cloud thickening. Northerlies. H 19 L 12
Wellington: A cloudy day, gusty northerlies. H 16 L 14
Queenstown Cloudy with showers. H 13 L 6
Wanaka Cloudy with sunny spots. Chance of rain. H12 L 8
Westport Cloudy with showers. Light winds. H 16 L 12
Nelson Cloudy with rain, light southerlies. H 16 L 13
Christchurch Cloudy, light easterly. H 16 L10
Ashburton Cloudy with light winds. H 15 L 8
Greymouth Cloudy with scattered showers. H 15 L 11
