MetService meteorologist Lewis Ferris said there was potential for an upgrade to a red warning - reserved for the most extreme weather events.
"This has the potential to be an extreme event and people are urged to keep up to date with advice from MetService in case Bay of Plenty needs to be escalated to red warnings."
Ferris warned people to expect 150 to 230mm of rain, though some areas may see more.
Streams and rivers may rise rapidly and flooding and slips were also possible, he said.
A "persistent" large band of rain was likely to sit over the Bay of Plenty but he said the intensity may be less than the Auckland downpour on Monday that caused widespread flooding.
"It is fair to draw some parallels but it is not going to be one or two hours of rain. It is more likely to be a longer event, with the potential for high-intensity rainfalls."
Ferris urged people to prepare for potential flooding and remove valuable items from their garages.
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Emergency Management Bay of Plenty spokeswoman Lisa Glass said it was important Bay of Plenty residents had an evacuation plan in place ahead of time.
She said people should prepare a grab-bag of essentials to take if they had to leave home urgently.
"You don't need to take the kitchen sink if you need to leave home - you just need to take something warm, something to eat, something to drink, your phone and all those basics," she said.
Emergency Management Bay of Plenty spokeswoman Lisa Glass. Photo / Emma Houpt
Glass also urged community members to check in on vulnerable friends and whānau who may need extra support or information.
Even people with Covid-19 should evacuate if needed, she said.
"Follow Covid protocols as much as possible, but don't let anything slow you down if you need to get out."
She urged people to prepare, take things as they come and avoid unnecessary panic.
"If everybody does their bit, and we all cross our fingers hopefully we will get through this without too much drama."
Rotorua Lakes Council said it was watching weather warnings and relevant teams were on standby to respond to any weather-related issues or emergencies.
The council urged motorists to take care on the roads as conditions could become hazardous.
Additional support crews from Fulton Hogan were also on standby.
During Monday's heavy rain, Tauranga City Council said it carries out checks when there is "significant rain" on the radar.
A heavy rain watch is in place for Taupō from 3am until 9pm tomorrow. Periods of heavy rain with possible thunderstorms are forecast.
There is a heavy rain warning from 9pm today to 11am Thursday in Gisborne and from 10am Wednesday to noon Thursday in Hawke's Bay.
A heavy rain watch for Auckland and Northland was due to be updated at 5pm today.
What to put in a grab bag
• Walking shoes, warm clothes, raincoat and hat • Water and snack food • Hand sanitiser • Phone and portable charger • Cash • Copies of important documents and photo identification • Medications, first aid kit, face covering or mask • Torch, radio and batteries
