Today the forecaster added Coromandel Peninsula to its heavy rain warning saying up to 130mm of rain could fall in the coming 24 hours.
It also extended its heavy rain forecast for Northland by another 12 hours.
Northland Civil Defence is now on high alert, monitoring the situation as the rain continues to fall.
It said there had already been 92.5mm of rain at Waimamaku west of Kaikohe, with a further 130mm of rain was expected to fall north of Kaikohe in the coming day.
Auckland passenger ferries across the Hauraki Gulf have been suspended for the second day.
The city is being told to brace for spells of torrential rain with an additional 100mm on rain on top of what has already fallen. The warnings apply mainly about eastern areas including the Hunua Ranges.
In Auckland a crash involving a bus and three cars on a southbound clip-on lane of the Harbour Bridge just after 9am has left traffic banked up for kilometres.
While there were no injuries the New Zealand Transport Agency said traffic was heavy from Constellation Dr and motorists were asked to be patient and expect delays heading into the city.
Overnight flooding in patches at the top of the South Island subsided after Nelson Police yesterday warned holidaymakers in low-lying areas to move to higher ground ahead of the forecast deluge.
MetService says Happy Sams in Tasman Bay had 333mm over a 24-hour period, including one torrential fall of 32mm in a single hour. All heavy rain warnings for the South Island have been lifted.
Duncan said the unpredictable weather system made it hard to pin down which areas would be hit worst.
"You get weird weather from tropical rain, you may get showers and 100mm in the same area.
"That is flooding criteria and it may be localised within a small area," Duncan said.
The warm front was being fed by airflow from Fiji and New Caledonia, and slowed by a high to the east, which would spare eastern areas like Gisborne.
"It will produce heavy downpours, it is a vein of tropical rain feeding in from the New Caledonia area."
Overnight heavy rain was to douse the upper half of the South Island, before moving to Taranaki and the western side of the North Island.
"The worst of the rain will hit Taranaki in the morning then move to a few places."
By lunch time it would pelt Northland and Auckland down to Manawatu, which was unusual, he said.
The silver lining amid the heavy rain was that strong winds across the country would begin to ease today, after peaking yesterday.
A wet weather warning is in place for Auckland from 6am today to 6am Friday. Between 80mm and 120mm could fall, MetService meteorologist Lisa Murray said.
Murray advised everyone to keep an eye on MetService reports to be best prepared.
Parts of the western ranges of Nelson had 100mm of rain already, Murray said.
Some areas were expecting 200mm to 250mm in the ranges as the front moved over, especially west of Motueka, Murray said.
"It is a slow-moving front and a lot of rainfall [is] associated with it. Some places could see 30 to 45mm heavy rain in an hour."
There was also a moderate risk of localised downpours of 20 to 40mm per hour from Northland to North Taranaki, and a general low risk of thunderstorms in those areas.
Thunderstorms were also possible about Westland, Buller, Nelson, northern Marlborough and from South Taranaki to Wellington during the morning.
A weakening front was forecast to move on to the far south of the South Island this morning, bringing a low risk of thunderstorms to southern Fiordland, Southland, Clutha and Dunedin.
MetService had severe weather warnings in place for Auckland, Northland and Taranaki.
Tomorrow the front would slowly weaken over northern and central parts of the North Island, where northerly rain is expected.
There was moderate confidence of rainfall reaching warning amounts from Waitomo to Northland and Coromandel Peninsula, and lower confidence over remaining central areas from Taranaki to Tongariro and Bay of Plenty.
Duncan said the tropical weather system was unusual and it could see parts of a region doused with 100mm of rain, while nearby areas might only get showers.
"It's a strange line of rain.
"Anything sub-tropical is unpredictable, where it will happen it is hard to know in advance, it bubbles in different areas," Duncan said.
Today's forecast
Whangarei: Heavy rain. Strong NE. High 24C Low 20C
Auckland: Heavy rain. High 23C Low 20C
Hamilton: Rain. Gusty NE. High 22C Low 18C
Napier: Cloudy and rain. N. High 24C Low 19C
Wellington: Rain. Strong NE. High 23C Low 18C
Christchurch: Fine. NE to S. High 30C Low 15C
Dunedin: Showers then chance of heavy rain and thunderstorms. High 23C Low 15C
