Metservice meteorologist Hannah Moes said the heat had been brought on by a warm northerly flow over the country bringing down warmer air from north of New Zealand.
"It is also aided by what is known as the fern effect where the air dries out and warms as it passes over the mountains so areas in the east will be warmer than in the west."
Ms Moes said the warmer temperatures were unusual for this time of year.
Tonight's warmer weather is not expected to stick around for long however. Cooler temperatures are forecast for many parts of the country tomorrow.
Ms Moes said highs of around 20-22C were expected in parts of the North Island tomorrow while temperatures of 17-20C were expected for the South Island.