It was not quite the coldest night of the year - last Wednesday, Pukaki in the Mackenzie district dipped to -7C overnight.
It was very cold in Central Otago last night too, with Ranfurly recording -6.3C deg, Lauder -6.2C and Clyde -5.9C, according to Niwa.
It gives "an honourable mention" to Auckland, where the temperature was 0.8Cdeg.
Niwa predicts an even colder start for most centres tomorrow.
MetService is also expecting frosts tomorrow with several centres, including Blenheim, Masterton and Taupō in the negatives this morning.
MetService said five more days of a ridge covering the country means people can expect chilly or foggy mornings and bright sunny days extending to the weekend; however, it said Wellington's notorious wind can continue to blow even under a broad ridge.
