WeatherWatch head analyst Philip Duncan says a series of lows in the Tasman Sea and New Zealand are linked to the tropical cyclone.
As a result, the next 36 hours could see some places receive up to 200mm of rainfall - particularly in the Bay of Plenty - and cause localised slips and flooding in both the North and South islands.
Those would be the highest rainfall amounts from this event anywhere in New Zealand, Duncan said.
"We are already being impacted by the rain - not directly from the storm, but connecting up to it."
The latest rainfall map showed the at least three separate low pressure zones connecting to the cyclone and which were taking away "energy" from it - in this case, heavy rain.
"That's why we're seeing this atmospheric river - all this rain coming out of the tropics. We saw flooding in Manawatū last night and we're going to be seeing more rain as that pushes down into the South Island," Duncan said.
A secondary low due tonight will bring a burst of more rain and strong winds into places like Northland and Auckland.
There is also the possibility of coastal gales.
Weather experts say the storm will eventually break apart while still near New Caledonia and Vanuatu.
Severe weather warnings and watches remain
A number of severe weather warnings remain in place in parts of the North Island and upper South Island; issued by the MetService.
Members of the public are advised to keep a close eye on local weather forecasts and updates in case of any changes made to the warnings.
An orange heavy rain warning is in place for Coromandel Peninsula, Bay of Plenty, Gisborne from Tokomaru Bay northwards, Tasman about and west of Motueka, the Tararua Range, Eastern Marlborough and the Canterbury Plains north of Ashburton - including the foothills and Christchurch.
There are also several heavy rain watches in place over Eastern Waikato and Taupō, Wellington, Kapiti and Horowhenua (not including the Tararua Range), the north and east of Northland and eastern Auckland - including Auckland City and Great Barrier Island.
