New Zealand can expect a warm start to autumn. Temperatures are expected to be slightly above average -- but make sure the coat is ready for a surprise seasonal cold snap.

MetService communications meteorologist Lisa Murray said it was likely the extreme heat and humidity that marked the final weeks of summer would be replaced by slightly cooler weather, but above-average temperatures were still expected for much of the country.

"We won't see the exceptional heat we've seen in February but March temperatures will be above average for the North Island and the upper South Island," she said.

This was related in part to extremely warm seas around the New Zealand coastline.


At the same time the lower half of the South Island could expect average temperatures this month.

However Ms Murray warned it was a volatile month where temperatures traditionally swung dramatically as cold fronts tracked up from the south.

"It's worth paying attention to forecasts when you're deciding what to wear in the mornings, because if you have a cold southerly coming through you will feel the nip in the tail of it," she said.

She said the country was still in the middle of tropical cyclone season and forecasters were monitoring developments around the clock.

"We're keeping a very close eye on it. It's quiet at the moment but we're keeping an eye on that space. There's still plenty of time for things to develop."

Ms Murray said the cloying humidity had been due to activity in the tropics.

"The really deep lows have been pushing humid air down over us and it's been sticking around for quite a while, because we've had these nice big ridges over us that don't blow it away."

However it did not necessarily mean the hot and sticky conditions would continue through autumn.

Niwa's seasonal outlook is forecasting similar above-average temperatures for the north and west of the North Island heading into autumn but likely to be near or above average for the rest of the country.

According to Niwa current El Nino conditions are likely to continue until April and then rapidly return to normal conditions or possibly change to La Nina conditions during spring between August and October.

El Nino conditions typically bring wet weather to the west, while drying out the eastern regions of New Zealand.