The west of the South Island is expected to face above average rainfall this summer, NIWA says.
It follows temperatures dropping to 10 degrees in the capital on Sunday, with downpours and blustery gales.
NIWA principal scientist Chris Brandolino told Morning Report the West Coast already had a lot of rain over the weekend.
"They're exposed to north-west winds and rain bands that'll becoming through the South Island off the Tasman Sea," Brandolino said.
"So if you have a summer holiday planned in Fiordland National Park, maybe the national parks along the west of the South Island, you might want to have heightened awareness of the weather, just because some of those rain bands could become quite heavy."
Temperatures in some parts of the country would be in the 20s but were expected to drop up to 12 degrees in some of the same areas, Brandolino said.
For the North Island, it was expected to be the opposite, with very little rain and long-term dryness around Christmas and the new year.
"Really we are going to see that most quintessential El Niño impact, in terms of dryness and in terms of warmth and that should last quite a while... I think just give it a few more days and we'll see more persistent dry warm weather," Brandolino said.
This weekend is expected to be hot in the eastern parts of the South Island and the North Island.
"Once we get towards Friday and beyond, it will be more sustained warmth and there will be no questioning where the warmth is, it will be well and truly here for much of the country, maybe not all but much," Brandolino said.
Some strong wind gusts were also expected to last up until Friday, including in Wellington, he said.
"It'll be a really windy week, I think this week is worth watching in terms of wind speed and fire behaviour, winds will be shifting around from north-west to south-west and when they shift to the south-west, that's when the colder air comes in."