It was a cold start for New Zealanders as both islands woke to frost and fog this morning.
However, the weather is expected to change to thunderstorms and potentially tornados as the country heads into the evening.
MetService has issued a series of severe weather watches but has not ruled out upgrading them to warnings.
MetService's Mt Cook Airport station recorded the lowest temperature overnight, dropping to -5.4C.
Taupō, Rotorua, Wānaka, Queenstown and Timaru were also below zero degrees at 7am on Thursday.
Fog formed overnight for many Northland regions, especially just inland away from coastal parts, WeatherWatch said. In the South Island, inland Southland, Otago and Buller experienced fog but it was less extensive than further north.
Frost was also recorded inland through both islands as far north as South Auckland on Thursday morning.
Heading later in the day, a low-pressure system over the Tasman Sea is expected to move slowly across the north of the North Island from Thursday night bringing periods of heavy rain.
MetService said localised downpours and thunderstorms are possible and has issued a series of severe weather watches.
A severe thunderstorm watch has been issued for Northland and Great Barrier Island from 9pm Thursday until Friday morning.
"A band of rain associated with an active trough is expected to move southeastwards onto Northland during Thursday evening. There is a moderate risk of thunderstorms with this rain band, which may spread as far south as Whangarei and Dargaville by midnight, then further south after midnight," MetService said.
"There is also moderate risk that these thunderstorms could become severe over Northland and Great Barrier Island, producing localised downpours of 25 to 40 mm/h, and possibly damaging tornadoes."
Heavy rain watches have also been issued for Northland, Coromandel Peninsula and parts of Auckland.
MetService noted there is uncertainty with the distribution and intensity of the heaviest rain and people are urged to keep up to date with the latest forecasts in case any of the watches are upgraded to warnings.
