Conditions are due to become more unsettled towards the end of the week as a frontal system goes over the North Island, he said.
"Warming up too in parts of Gisborne, Hawke's Bay and Wairarapa with forecast maximum temperatures in the mid-twenties."
Aucklanders are experiencing a high of 21C today and an overnight low of 15C.
A WeatherWatch spokesman said: "The warmer than usual run of weather is being caused by a sub-tropical or northerly flow which is spreading down over the North Island and some parts of the South Island."
Inland parts of the South Island are showing over 8 degrees above normal temperatures by day.
Although some coastal areas are reporting a cooler sea breeze, many places inland are warmer than usual as a result.
"The mild run of weather continues on this week even as a low in the Tasman Sea moves through then falls apart," he said.
Temperatures will see a marked drop later in the week, with Invercargill set to see a shift from a high of 21C on Saturday to 11C the next day.
Alexandra will from a warm high of 22C on Friday to 14C on Sunday.
"For many, the cooler Sunday and Monday change will only reset temperatures back to normal for this time of year."
