"Rain does move to the North Island on Sunday, with the heaviest rain likely in the southwest and east generally speaking," WeatherWatch analyst Phil Duncan said.
"The upper North Island will continue to see high humidity...through all of next week.
Basically for the next 10 days out the dewpoint doesn't drop below about 17 or 18 degrees, it reaches 21 or 22 on Sunday, which is very humid."
Auckland was expected to have an overnight minimum temperature of a warm 21C or 22C, he said.
MetService is forecasting only a chance of a shower in Auckland on Saturday with a high of 28C.
This is followed by slightly cooler highs and the occasional rain on the following two days of the long weekend.
Wellington is forecast to have rain, with heavy falls on Saturday. On Sunday the forecast is for rain, possibly heavy, but expected to clear on Monday.
Southerlies arriving early on Saturday will cool the temperatures, and 16C is the forecasted high for Sunday.
On Saturday, rain is expected to ease to occasional showers in Christchurch with southeasterlies taking the high to just 15C.
On Sunday, cloudy skies are expected, and occasional showers and fine spells increase on Monday.
MetService communications meteorologist Lewis Ferris said humidity was mentioned in forecasts for Northland, Auckland, Bay of Plenty and Waikato from Saturday through to Monday.
The high temperatures was the result of a funnel of air that had been pulled down from the tropics this week.
"This is a little bit out of the ordinary," Ferris said. "It's more about the extremity of those temperatures."
Low temperatures are forecast to be 21C overnight Friday and Saturday, 20C overnight Sunday, and 19C overnight Monday for Auckland.
Ferris said the average overnight low for this time of month was normally around 16C.
But the heat experienced in Auckland is nothing compared to the sweltering 32C forecast high for Hamilton with humid northeasterlies tomorrow, before dropping to 27C and 24C on Sunday and Monday.
