MetService says the humid conditions causing Auckland and Northland residents grief are likely to remain in place for two more days.
MetService forecaster Marylin Avery says all of Northland and Auckland experienced humidity of up to 100% on Tuesday night.
People in Whangaparaoa Peninsula north of Auckland, and Awhitu, southwest of the city had endured the maximum humidity level for most of the previous night.
Auckland reached 95% humidity at 1am on Tuesday and Whangarei 96% for much of the early morning.
Ms Avery says muggy conditions will remain in place in Northland and Auckland for two more nights, before an expected wind change on Friday.
MetService says such high levels occur just once or twice a year when high temperatures combine with moist tropical air.