Temperatures are forecast to be warmer than average in the North Island and most of the South Island until the end of the year.
NIWA meteorologist Ben Noll said NIWA's seasonal outlook suggested temperatures in October could be variable and a bit cooler before a marked rise in November and December, along with an increase in humidity.
"As we go into November and December La Nina really beds in, sets up shop and I think the cooler temperatures will really go out the window as we look to the second half of the upcoming three months," Noll said.
Noll said until September there were above average temperatures every month this year.
"But it's looking like September is going to buck the trend ... in fact we may have nationally near-average temperatures for the first time since last September."
The last four Septembers had been more unusually cold than the preceding traditional winter months of June, July and August, Noll said.
Normal rainfall is expected across most of the country in the next three months, although occasional sub-tropical lows could increase the risk of heavy rain in the north and east of the North Island.
There have been moderate La Niña conditions during September. A marine heatwave has strengthened in New Zealand's coastal waters with sea surface temperatures up to 1C above average this month.
RNZ
