Civil Defence in regions throughout the island are closely monitoring the developing storm, which threatens to bring widespread flooding.
Taranaki, Whanganui, Hawke's Bay and the Tararua Ranges will be the first to experience the worst rain. All districts are under heavy rain warnings, expecting up to 400mm in parts over the next 48 hours. Downpours are expected to become very heavy today.
The MetService warned these areas would experience intense rain for the next day and a half.
The rain may cause flooding and slips and people are being warned to watch for rapidly rising rivers and streams.
Severe gales are also expected to lash southern Taranaki through to Buller from today and overnight.
This morning the MetService rain radar showed the storm making landfall and widespread torrential rain falling across western regions of the lower North Island. Up to 70mm of rain had already been recorded across central districts in the past 12 hours.
Taranaki Civil Defence said no roads were reported closed but officials were monitoring the weather.
Central District Police are asking people to drive to the conditions and follow at a safe distance.
Horizons Regional Council hydrology and emergency management staff were keeping an eye on river levels across the region overnight.
Niwa says the remnants of Tropical Cyclone Debbie are threatening to develop into a serious situation in the next 72 hours.
Tropical downpours are set to saturate Northland and Auckland tonight and Wellington will be drenched by heavy falls tomorrow.
Astounding levels of rain are forecast to saturate the North Island: 125mm is to fall in Northland, Taranaki and the central plateau by tonight.
In the latest MetService warnings, Whanganui is expected to get 250mm of rain in 33 hours from this morning.
"This is a significant amount of rain for these areas and people are advised to watch out for rapidly rising rivers and streams, and possible surface flooding and slips," said the MetService.
Regions from Coromandel to Nelson have been placed on a watch, and heavy rain is likely to reach warning levels by tonight.
Severe gales are also expected to lash southern Taranaki through to Buller.
Niwa forecaster Ben Noll warns the deluge will be so bad it may drop up to three times the month's normal rainfall n just three days.
The MetService said the worst weather will hit today and tomorrow when heavy falls are expected across central New Zealand. Strong southeasterly winds could also turn into destructive gales.
WeatherWatch said central regions around or near Cook Strait, and those facing the Tasman Sea, such as Nelson and Taranaki, were most at risk of flooding.
The Ministry of Education is advising parents to stay in touch with schools and early childhood services.
The ministry said the best information would come from schools as they decided this morning whether to close because of flooding.
Hot and muggy
But the wet weather will bring hot and muggy conditions with it, particularly for those in the north of the country.
A combination of low cloud, fog and mist cloaked western and inner-city Auckland suburbs in a thick white blanket this morning, shrouding high-rise buildings in downtown Auckland, obscuring landmarks such as the Sky Tower.
Auckland is in for hot and sticky nights this week, and the overnight minimum for today and tomorrow is expected to be 19C.
The temperature is expected to drop to 16C tomorrow night, which MetService meteorologist Lisa Murray said would be "slightly more pleasant for sleeping".
"It's really about comfort levels for humidity and different people can handle more of it than others.
"If Dunedin got the humidity Auckland gets, I don't know how many people would manage very well because they're not used to it, but in Auckland most people can hack a bit of humidity."
Murray said hot and humid conditions were typical for autumn, as the last of the summer warmth lingered before winter set in.
"This time of year it's still quite humid, the next few days it's going to be warm and muggy."
