Raincoats look to be the "must have" accessory of Christmas Day, with MetService predicting even Santa will be wearing his as he navigates south and central New Zealand.
And it will also be damp in the north of the country.
MetService duty forecaster Daniel Corbett said the complex weather system moving over the country today was making things "a bit messy".
"It's been spreading heavy rain into the West Coast of the South Island and the front is slowly moving up from about Westland to central New Zealand," he said.
"By the end of today it's bringing strong northwest gales ahead of it, so winds will be strengthening through the day in places like Wellington, with winds easily getting up to 100kmh here.
"The heavy rain on the West Coast will also work its way north, so for instance the Tararuas in the lower North Island could see some heavy persistent rain in the later parts of today."
The front looks set to slowly move north, until it sits across much of southern and central New Zealand.
"There will be some thunderstorms too across western parts of the South Island down to the far south, Corbett said.
"The whole weather system moves north during the night so you can imagine by Christmas, Santa is going to have his raincoat on, and he's probably going to have the reindeer moving at full speed because the winds are going to be quite strong."
A damp start to Christmas Day is expected for the top of the North Island - from Auckland to Hamilton down to the Bay of Plenty.
"The front will have moved through places like Wellington, there will still be some showers on the West Coast . . . but through the day it does start to slowly dry out in places.
"What is a damp start to the day in places like Auckland, and Bay of Plenty, improves there, with a cooler fresher southwest flow."
Showers will hit the west coast of the North Island and the southeastern coast of the South Island meaning the Otago and Canterbury regions will get "a bit of wet weather" during the course of Christmas Day, Corbett said.
"So no one is completely dry during Christmas Day," he said.
"Some spots are drier, and all of us will manage at least some dry spells, but raincoats will definitely be coming in handy."
The warmer spots for Christmas Day look to be the Hawke's Bay, and the far north of the North Island, where temperatures could sit in the low- to middle-20s.
But down to the far south and east of the South Island, some spots will struggle to get into the mid-teens for the afternoon, Corbett said.
"This low sits and lingers through Boxing Day - the far south and west of the country does a little bit better, it's a little bit drier, and as we get into Friday and Saturday the low is easing away and drier weather starts to return."