The high was forecast to bring a northeasterly front over both islands on Tuesday and Wednesday, and a band of rain would develop on Wednesday.
"I don't see the rain as being too much of a problem but there might be a couple of isolated thunderstorms and some heavy downpours in the upper North Island on Wednesday with gusty winds," Duncan said.
"It doesn't look like a storm at this stage but it could be on the threshold of maybe a rain warning and certainly some marine wind warnings."
MetService meteorologist Claire Flynn said inland areas were in for another chilly night tonight; -1C was forecast for Taumaranui, Taupo, Tokoroa and Timaru, and Alexandra was set to hit -2C.
Yesterday's daytime high was 17.6C in Kaitaia and the low was 6.9C in Mt Cook.
TEMPERATURES:
Auckland: High tomorrow: 16C. Overnight low tonight: 7C
Hamilton: High tomorrow: 14C. Overnight low tonight: 0C
Napier: High tomorrow: 15C. Overnight low tonight: 1C
Taupo: High tomorrow: 11C. Overnight low tonight: -2C
Wellington: High tomorrow: 14C. Overnight low tonight: 6C
Blenheim: High tomorrow: 14C. Overnight low tonight: 0C
Christchurch: High tomorrow: 13C. Overnight low tonight: 1C
Queenstown: High tomorrow: 10C. Overnight low tonight: 0C
Invercargill: High tomorrow: 12C. Overnight low tonight: 2C