A big low brewing in the Tasman Sea is set to siphon moisture from near Papua New Guinea and send it to New Zealand, hitting the upper North Island with "copious rainfall".
"Check out this tropical connection! It will bring heaps of moisture to northern New Zealand later this week," NIWA says on Facebook, along with a graphic showing the moisture's movement.
"The result will be copious rainfall in the north, including Auckland - consistent with a La Niña-like atmospheric pattern."
MetService says to expect a wet week with bands of rain lasting right into the weekend.
"A new low system will take control of most of the country's weather from Wednesday with rain and blustery northeasterlies forecast to sink south over the upper North Island," it says.
"Rain with this front is expected to be prolonged for many areas due to its very slow movement south during the second half of this week."
Elsewhere, eastern coasts will be affected by this low system as developing northeasterlies mean cloudy days and scattered light showers.
"Cloud over these areas will be bad news for those looking to observe Matariki but all is not lost as conditions look good for those who are able to head to the South Island ski slopes," MetService says.
"The western coasts of the South Island are looking to hold their 'sunshine coast' title from last week with generally dry and fine conditions forecast there this week."