Mr Marsters said "the cold stuff" formed just before sunrise when the air temperature dropped.
"In previous days, there has been some cloud cover and coastal breezes coming in.
"Combine clear skies and calm night conditions with a big high pressure system parked over the North Island and it's ideal conditions for chilly temperatures."
Sunday morning was a cold one too with Whangarei officially recording 3C, Kaitaia dipped to 4C and Kerikeri was slightly above freezing on 1C.
There were no temperature readings for Dargaville as it appeared there was only a rain recorder and no temperature sensor at the MetService site.
If you thought it was cold in Northland yesterday spare a thought for those at Mt Cook Airport where MetService recorded -8C. Lake Tekapo dipped to -8 and Alexandra -7.
In the North Island Waiouru was -7, Taupo -5 and Rotorua -3, as was Paeroa.
The perfect conditions for frost to form in Northland were expected to disappear overnight Tuesday and give way to some wet weather on Wednesday.
"Cloud comes in from the west and a low crosses Northland. There will be a chance of heavy falls."
But the good news is the weather should improve just in time for the weekend.
Earlier in July, Kaitaia got down to 3.9C and Kerikeri was down to 5.8C.
And while it seemed cold in Northland this past week, it wasn't as cold as our coldest day last year. Last July 1, the official overnight low in Kaitaia hit 0.3C, Whangarei had an overnight low of 1.3C, Kerikeri's was 1.3C.
