"When the winds ease back, that cold air will sink into the valleys. The high will basically lock in the cold air for the rest of the working week".
WeatherWatch.co.nz said frosts would mainly stick to sheltered inland places at this early stage with eastern parts of the North Island, mainly around Gisborne, ruled out from frosts this morning due to a southerly wind and passing clouds with the odd shower.
MetService meteorologist Micky Malivuk said the cold snap would continue through to next week. "Apart from the Gisborne, Manawatu and Wanganui regions, [there will be] clear, cloudless mornings, which mean frosts for most of the country.
"Central Otago and the central plateau in the North Island will most likely have the lowest temperatures."
Mr Malivuk said a low developing in the Tasman Sea would move towards New Zealand. He expected the worst of the cold snap to have eased by Tuesday because of the low and an incoming northeast flow.
MetService said the low temperatures were not unexpected for this time of the year.