Record temperatures in the high 20s Celsius were seen today, but MetService is forecasting an "abrupt midweek plunge" will see temperatures cut in half around the country.
Napier at 28.1C and Hastings at 27.8C appear to have today registered record temperatures for May.
But the forecast cold snap is set to hit after what MetService called a warm, wet and unsettled week. It said the weather change would arrive in "true autumn fashion".
Napier's high is expected to be just 15C on Thursday.
MetService meteorologist Alwyn Bakker said: "The difference will be most notable in the South Island, with a few locations set to see around a ten degree drop in temperature.
“In particular, Christchurch is expected to reach 24C on Tuesday, just ahead of the change, but only 11C on Wednesday.”
A cold southerly change sweeping up from the south is responsible for the chilly weather.
When it hits the North Island, MetService says the overnight temperatures will be most affected.
"Auckland will be going from a muggy 16C on Tuesday night to a mild 8C on Wednesday night."
The cold front will also bring snow to parts of the South Island.
"The snow is expected to lower to 600 metres in Southland, Dunedin, and the Southern Lakes from Tuesday afternoon, and to 700 metres at night in the Canterbury High Country. Road Snowfall Warnings have been issued for high level roads in the areas," MetService says.