The top of the South Island also remains under a warning. Nelson and the Marlborough Sounds will be inundated by rain until mid afternoon.
Other regions, including Auckland, are under a watch, as rain will fall incessantly for the best part of today before starting to clear this evening.
Rivers are starting to rise in Northland as intense rain falls across the top of the country but at this stage none are threatening to burst their banks.
"The story of the day is rain," said forecaster Ravi Kandula.
"The rain band over the North Island is expected to intensify during the day."
The most intensive falls would hit Bay of Plenty and the centre of the island during the morning and would not let up until late tonight.
Some regions are preparing for staggering amounts of rain. In just 24 hours Mt Taranaki was expected to see 180mm and Tongariro National Park 160mm.
But in the first 12 hours some totals had already been exceeded, as MetService has recorded 185.5mm of rain near the mountain.
Heavy rain is falling across the Bay of Plenty, but the Bay of Plenty Regional Council says it is keeping a careful eye on the situation and there's currently no cause for alarm.
The Eastern Bay will be most affected by the forecast rainfall. Metservice is forecasting 60-80mm of rain in areas west of Whakatane from midday to 10pm today. Areas east of Whakatane are expected to receive 70 to 100mm from 1pm to midnight, especially about the ranges.
This amount of rain may cause rivers and streams to rise rapidly, and surface flooding and slips are possible. Driving conditions could also be hazardous, so people are urged to take care on the roads.
Regional council duty flood manager Roger Waugh said while some rivers may get to their first warning levels there was no cause for concern for any major flooding. First warning levels generally mean farmers in low-lying areas move stock to higher ground.
In the Rotorua lakes region, there may be a small increase in Lake Okareka's levels but everything is in place to manage any issues, the council said. The Okere Gates are fully open for their annual drawdown and will remain open for the duration of this event.
Regional council staff and Bay of Plenty Civil Defence are monitoring the situation and will provide regular updates.
Kandula said up to 20mm fell in Taranaki overnight but the region was set to be inundated by another "significant" amount of rain this morning.
MetService has also issued a severe weather watch for several other parts of the country, including Coromandel Peninsula.
The watch means there is a possibility of rainfall reaching warning levels of 60-70mm within 9 to 12 hours, or 85mm in 18 hours from early Wednesday morning.
West of Whakatane, rainfall was expected to become heavy from midday tomorrow and ease late tomorrow night.
As well as the rain, strong northerly winds are expected to buffet parts of Northland, Auckland and Bay of Plenty.
Gusts of up to 100km/h are expected in gales lashing Auckland and Great Barrier Island this afternoon.
Kandula said eastern parts of the North Island would be the only regions to escape the coming deluge.
Meanwhile, fog in Christchurch has disrupted three domestic flights in and out of the city this morning.
A return flight between New Plymouth and Christchurch has been cancelled and a flight from Auckland to Christchurch has been delayed.
