Summer rain to cool country after record-breaking temperatures

For those struggling to sleep in the sweltering summer nights, relief could finally be in sight.

Wet weather is on the way for much of the country, which MetService says will bring down record-breaking temperatures.

A "rain-laden frontal system" moved over the lower South Island on Sunday morning and is expected to travel up to central New Zealand by evening.

A heavy rain watch was issued for Westland and Fiordland, while a strong wind watch was in place for western Southland including Stewart Island between 9am and 11am.

Thunderstorms are possible in several areas, with MetService predicting a moderate risk for the West Coast of the South Island and the central North Island.

A high pressure ridge heated up the country in the last several days, with Auckland and Wellington seeing higher than normal January averages.

"Napier got to 33degC on Saturday which is very close to the highest January temperature on record for that area," says MetService meteorologist Mark Bowe.

The ridge will move off to the east on Sunday, providing some relief from scorching days and muggy nights.

North Islanders are advised to keep an eye on weather updates going into next week, with a trough bringing rain to much of the island from Monday morning.

"Plenty to keep an eye on as changeable weather is expected for early next week," says Mr Bowe.