A cold front due to move up the South Island is expected to bring the biggest frosts of 2020 - and some areas will plunge to as low as -5C in the weekend.
A weak front will spread northwards up New Zealand on Thursday and Friday, and this will be followed by a colder southerly flow that's "locked in" at night, WeatherWatch forecasts.
A high exiting Australia will merge with another over New Zealand to become one powerful high. This system will keep cold air stuck in inland valleys and will see some of the biggest frosts this year.
"For the most part the South Island will be smack bang under the centre of this weekend high," WeatherWatch says.
"So the cold southerly change at the end of this week will be locked in, seeing overnight lows plummet to possibly -5C through well inland areas like Tekapo."
Queenstown, Alexandra and Cromwell have current overnight lows between -1C and -3C, but areas near these could also dip to -5C, creating heavy frosts.
Coastal areas of the South Island won't feel the cold of this weather system as much, with wind and cloud cover keeping frosts limited. On Saturday, Christchurch and Dunedin will reach 9C, and more northern areas may reach 12C.
The North Island is expected to be milder this weekend and places north of Waikato should see morning temperatures between 6C and 10C. This will rise to 18C for parts of Northland and 14C for Hamilton.