It is expected to be a warm spring overall, but with some cool temperatures this month.
NIWA's seasonal outlook through until November picks long, dry spells punctuated by heavy rainfall events.
A sub-tropical low during the first week of September may elevate the risk for heavy rainfall in the northern North Island.
Another sub-tropical low is possible in the third week of the month and may bring more rain, possibly heavy, to parts of the country.
It expects warmer than average temperatures for all regions, and continuing marine heatwaves for many places.
Rainfall is likely to be around about normal overall.
A rare third straight summer of La Niña conditions are expected, likely bringing heavy rains in the north, and drier conditions in the south.