A massive low that built momentum in the Tasman Sea has caused slips and surface flooding.
By midday, 90mm of rain had fallen at New Plymouth Airport.
"Avoid unnecessary travel, drive to the conditions, and watch out for surface flooding."
"This is an evolving situation."
High-tide, expected later on Sunday morning, poses the highest risk.
A low-pressure system has caused gale-force winds and huge swells are expected for Wellington.
MetService recorded just 67 hours of sunshine in Kelburn this month.
"We're not even at the peak of our sea surface temperatures."
People are advised to keep up to date with the latest forecasts.