Brisk temperatures and consistent rain is set to plague the first week of school holidays.
The national high for today is 19C, but that drops to just 14 on Tuesday as a low sets in.
Metservice meteorologist Curtis Hayes says the South Island will be the chilliest.
"Overnight lows around 1C to 2C in a lot of places in the South Island. By Tuesday/Wednesday, the daytime highs will be 11C to 15C."
A heavy rain warning is in place for Westland, south of Hokitika, on Sunday.
"We have a high in Christchurch of 22C, but by Tuesday that's dropped to 11C - so it'll be quite a shock," says Hayes.
Some places such as Alexandra will hit 0C overnight, NIWA says.
It's not predicted to fine up again until Friday, so parents should be prepared to keep their children entertained indoors for the first half of the holidays.
"Ahead of this low and as the low is crossing it'll be wet, but still mild," says Hayes. "Monday night is when everything's going to change for most people."
"The warmer weather is ahead of a cold change tomorrow and Tuesday spreading in nationwide - so make the most of today," the privately owned forecaster warned on Sunday.
Snow will fall to 300m in southern parts of the South Island, MetService says, possibly enough to "cause stress to livestock and affect higher roads and passes".