NIWA is predicting the start of winter will be warmer than usual, after a "last hurrah" of unsettled weather this week.
Principal scientist Chris Brandolino said after an unusually chilly May, temperatures could "flirt with 20 degrees [Celsius]" in the eastern parts of both the North and South Islands this weekend.
In Christchurch, daily lows could be 10 to 12 degrees warmer than they will be today and tomorrow.
Brandolino said the mild weather will stay for some time and is likely to be a theme of winter.
"I think this pattern shift kind of marks something that may be a bit more persistent," he said.
"As we look into the first sort of couple of weeks, 10 days maybe, of June, I certainly see more days that are going to be unusually warm versus unusually cool."
The lead-up will be more unsettled, however, with a polar jet stream continuing to bring below-average temperatures and southerly winds as autumn ends.
"Wednesday looks pretty windy across the country, particularly for the upper north," Brandolino said.
The winds will die down as the air begins to warm on Friday.
Lack of rain could be a concern
NIWA's drought forecasting shows the next month will be drier than normal for most of the country, which Brandolino said will be of concern if it continues through the winter.
"It's really important that this time of the year we get the water that we need, because if we don't, we'll basically walk into another warm season or growing season kind of behind the 8-ball."
NIWA's data shows the ground is currently unusually dry for this time of year in the upper and eastern South Island, and lower North Island.
NIWA is expected to release its seasonal outlook for winter on Thursday.
