Temperatures for the twin cities are forecast to begin rising from 16C today, 18C tomorrow and then into the minor midwinter heatwave of Thursday and Friday.
Western regions of the North Island will receive rain, although temperatures are expected to be relatively mild.
There is also a "moderate probability" of warning amounts of heavy rain about Mount Taranaki and the Tararua Range from Tuesday to Wednesday, and a low confidence for eastern Bay of Plenty on Wednesday.
While there is rain forecast for the Bay, it is not expected to arrive until late Saturday.
Weekend temperatures will remain in the mid to high teens but the glimpse of spring will dissolve next Monday when the winds return to more winter-like southerlies.
Southwesterlies, showers and 12C temperatures will kick next week off.
If this Thursday's forecast 20C does emerge, then it will equal the mid-summer temperature posted in Dublin yesterday, and will be just one degree short of yesterday's top temperature in Noumea.
The mainly mild winter so far has led to early splashes of colour appearing on roadside fruit trees in parts of Napier and Hastings.
Trees in Westminster Ave in Tamatea began to start blossoming about a fortnight ago, one resident said.
WeatherWatch forecaster Philip Duncan said he was in no doubt the spring of 2014 was under way.
He said autumns and springs ran for four months each, and winter and summer two months each - and winter had had its run.
"Forget the meteorological calendar, which says winter is here for another four weeks. Forget the astronomical calendar, which says winter is here for another two more months."
He said the best way to assess the seasons was by observing nature itself.
"Spring is when this life comes back again - when buds start to bloom, daffodils flower, lambs and calves are born."
