NIWA warns the odds are growing for a "significant rain event" next week as an "atmospheric river" moves south and teams up with low pressure.
The forecaster's graphics show a flow of subtropical moisture heading down through the Tasman Sea and directly impacting the West Coast of New Zealand early next week.
This slowly moves up over the South Island, driving strong northerly winds and rain to many regions on Wednesday and affecting the west of the North Island on Thursday.
NIWA said the risk of flooding is a concern with parts of the South Island currently facing the greatest threat.
"Areas along and west of the Divide (Alps) have the greatest odds," it said on Facebook.
"Depending how much rain spills over, some of the headwaters as well as interior Otago could also be at risk. The jury is still out on this.
"Heavy rain will shift north as next week unfolds with the North Island getting in on the action Thursday and/or Friday."
MetService's severe weather outlook for Wednesday shows moderate confidence of heavy rain in northern Fiordland and the Westland District, extending into the headwaters of Otago and Canterbury lakes and rivers.
Confidence increases to high for heavy rain in central and southern parts of the Westland District.
There's also low confidence for severe north to northwest gales about exposed inland parts of the South Island from north Canterbury to central Otago.
On Thursday, as the strong and moist northerly flow continues to affect the South Island and starts to affect the west of the North Island, there is moderate confidence for heavy rain about the Westland District, including spillover into the headwaters of the Otago and Canterbury lakes and rivers.
For the Grey District, Buller, the northern and western Tasman District, and northern Fiordland, confidence is low for heavy rain.
There's also low confidence for severe northerly gales in Taranaki, Wellington and northern Marlborough on this day.
Newshub.
