A severe thunderstorm watch is in place for the Western Bay of Plenty region, with potential for heavy rain and possibly a coastal tornado.

The MetService issued the warning for the Western Bay of Plenty, Coromandel Peninsula, Waikato, Northland, Auckland, Great Barrier Island, Waitomo and Taranaki.

Overnight from 8pm, 17.2mm of rain fell in Tauranga, 15.8mm in Rotorua, 16.5mm in Ōpotiki and 29.2mm in Whakatāne as the thunderstorm watch "intensified a little", and a rain band moved over the area, a Metservice forecaster said.

Of the 29.2mm rainfall in Whakatāne, 12mm fell in one hour from 4am.

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The thunderstorm watch is expected to be cancelled at 9am today but there is a risk another watch will be issued for the eastern Bay of Plenty for this afternoon and evening.

MetService forecaster Andy Downs said scattered thunderstorms were expected about northern parts of the North Island overnight Wednesday to early to mid-morning today.

"There is a risk that some of these thunderstorms could become severe from North Taranaki up to Northland and across to the Coromandel Peninsula and Western Bay of Plenty.

"There was the potential for localised very heavy rain or downpours of 20 to 35mm per hour and possibly a small coastal tornado," Downs said.

"Rainfall of this intensity can cause surface flooding and may also lead to slips and hazardous driving conditions. If any tornadoes occur, they will only affect very localised areas," he said.

People in these areas should be on the lookout for threatening weather conditions and monitor for possible severe thunderstorm warnings.

For information on preparing for and keeping safe during a storm, see the Civil
Defence Get Ready, Get Thru website.