However, a southerly change will soon make its way up the South Island.
That will bring down temperatures tomorrow.
The cool weather will be short-lived, with a ridge of high pressure expected to return the warm summer conditions to most of the country by the middle of the week, Mercer said.
The weather today shouldn't cause any major problems for major events taking place today, although those heading to Auckland's Christmas in the Park might want to pack an umbrella.
Auckland will be fine and warm most of the day, with a high of 24C, but there is a chance of a shower in the evening.
It won't be cold out though, the temperature is expected to still be in the low 20s when the concert begins at Auckland Domain at 7.30pm.
A warm day is also forecast for Hamilton, where the second cricket test between the Black Caps and the West Indies begins at Seddon Park at 11am.
Sun protection will be needed today with a high 25C and cloud burning off, but showers are possible later in the day, Mercer said.
Highs today
Whangarei: 24C
Auckland: 24C
Hamilton: 25C
Tauranga: 23C
Napier: 30C
Wellington: 22C
Nelson: 24C
Christchurch: 31C
Dunedin: 29C
Queenstown: 24C