"If heavy rain occurs during rush hour, expect significant travel impacts," the forecaster warned - but added that the weather was likely to blow through by then.
But MetService forecaster Tom Adams said the chance of thunderstorms was regarded as low at this stage.
"We are currently giving that a low risk but we'll be reassessing watches and warnings in coming hours," he said.
Rain had been falling steadily since 11pm in Northland and warnings remained in place for the coming day.
"The main front is yet to come," noted Adams.
So far the northern provinces had escaped intense downpours which threatened to cause problems across the water-logged region.
"It's a wet morning in Northland and that will continue throughout the day," said Adams.
The forecaster broadened the span of watches to include the Bay of Plenty and northern Hawke's Bay with heavy rain expected to deluge both regions across the day.
Meanwhile, motorists were being urged to take care while driving across Northland this morning.
The New Zealand Transport Agency said with the ground still saturated from the floods last month, heavy rain was likely to see rivers and streams rise rapidly.
It warned that driving on the region's roads could be hazardous and pleaded with drivers to drive to the conditions, staying alert for surface flooding, slips and fallen trees or branches.
Civil Defence Northland this morning said rainfall totals so far had been well within what the region was able to cope with.
The Northland Regional Council's rain gauge on the Aupouri Peninsula in the Far North recorded 31.5mm over the past 12 hours and all others less than that. At this stage, rivers were only slowly rising.
Motorists were urged to take care on the roads, with localised surface flooding and slips remaining a possibility.
