NIWA says New Zealand is in for "several spells of unseasonable warmth" over the next three months.
It's issued its seasonal climate summary for May - July on Wednesday, giving Kiwis a glimpse of what is expected to come.
During April, marine heatwave conditions continued in the Tasman Sea, and warmer than average sea surface temperatures are forecast to persist during the coming three-month period.
This is expected to contribute to spells of "unseasonable warmth" and invigorate low pressure systems as they approach New Zealand from the west.
"Warmer than average coastal and Tasman Sea surface temperatures may influence several spells of unseasonable warmth through the season, particularly in eastern areas, contributed to by frequent westerly air flows," NIWA says.
"Warmer than average seas may lead to a reduction in the intensity and duration of cold spells during the coming season."
Temperatures are forecast to be above average in the north and east of the North Island and east of the South Island, and about equally likely to be above average or near average in all remaining regions.
However, cold snaps and frosts are likely to occur, particularly in colder locations, as the season progresses.
At the same time, rainfall is likely to be either below or near normal in the North Island and in the east of the South Island and likely to be above normal or near normal in the west of the South Island.