A polar jet stream is set to bring a chill across most of the country as winter nears.
"Autumn is ending as it started - with below-average temperatures," Niwa says.
"Sunday looks to have a wind chill across most of the country which may be reinforced toward the middle part of next
week."
It said a polar jet stream was heading towards the motu this weekend.
But milder temperatures are forecast heading into June, as the "polar jet" is expected to calm down.
Niwa said with El Niño ending, "its influence on the jet stream patterns is expected to wane".
It said winter may bring more westerly or northwesterly winds - "a stark contrast to autumn's chilly southwesterlies".