Despite a misty start to Tuesday in Whanganui, the early morning Air Chathams flight to Auckland departed on time.

Fog did affect the first incoming flight however and the arrival was delayed by an hour.

The morning mist is the precursor to some more balmy weather on its way in September.

National Institute of Water and Atmospheric Research (NIWA) forecaster Ben Noll says the winds flowing south from near New Caledonia at the end of this week will at times make it feel more like November or December.

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The forecast means that there may be a couple of rounds of record-breaking maximum and minimum temperatures in September.

"We are forecasting unseasonable warmth on Thursday and Friday across the east of both islands, where temperatures may rise to between 5 and 10°C above the average maximum daily temperature.

"Some places may approach 25°C as a classic foehn [warm, dry southerly] wind develops," Mr Noll said.

Winter temperatures in Whanganui on some days have been warmer than average with unseasonably warm days in June and July.

"The temperature in Whanganui on June 11 was 19.8 which is 5.7 degrees above the average of 14.1 and the temperature also reached 19.8 on July 24 which is 6.5 above the average for that date," said Mr Knoll.

The warmer temperatures may have been pleasing for some but disappointing for snow sports fans as the mild weather may also have contributed to some ski field snowmelt, especially at Mt Ruapehu.