Wellington, the Nelson Tasman region and the West Coast are being warned to get ready for Cyclone Gita, which is expected to pack a punch when it hits the country tomorrow.
The cyclone is expected to approach New Zealand from the north-west as it transforms into an extra-tropical cyclone tomorrow or Wednesday, MetService says.
Waves in excess of six metres could hit due to high tide overnight tomorrow causing coastal inundation - while heavy rain will cause slips, rising rivers and flooding.
Ms Ardern told reporters in Wellington today that the cyclone was being watched closely and estimates were being made by Civil Defence and MetService in terms of its degree of impact.
She was to meet Civil Defence minister Kris Faafoi, Climate Change minister James Shaw and associate Transport Minister Julie Ann Genter this afternoon at MetService headquarters to get a briefing.
"We're tracking its [Gita's] progress to try and get a sense of the magnitude and make sure there is a co-ordinated response across areas it will affect," Ms Ardern said.
Heavy rain warnings, including severe gales, are in place for Nelson and Buller from 9am tomorrow, with 150-200mm of rain expected to fall west of Motueka.
The Ministry of Civil Defence and Emergency Management is advising people to prepare for the possibility of power cuts, water outages and road closures.
Meanwhile a weather warning for Wellington starts at 1am tomorrow and Marlborough - including the Kaikoura coast - from 10am.
Wellington Mayor Justin Lester says commuters should make early decisions tomorrow morning about whether to stay home or delay journeys.
All schools in the Buller and Grey districts are shut for the next two days, but a decision on the declaration of a State of Emergency will be delayed until tomorrow.