Some parts of the Western Bay could get up to 150mm of rain today, forecasters say.
Niwa principal scientist in forecasting Chris Brandolino said heavy rain was forecast as the remnants of Cyclone Debbie passed over New Zealand, siphoning warm wet weather from the tropics with it.
"It makes it a very rich, moisture-laden atmosphere which will produce heavy rainfall," he told the Bay of Plenty Times.
There was some variability in total rainfall, but most areas could expect between 50 to 100mm with others getting localised downpours of up to 150mm between 6am and 9pm today, he said.
April would only normally get 120mm of rain for the whole month.
"In the 24 hours ending Wednesday at 8am it was possible a month's worth of rain would fall," Mr Brandolino said.
There would be a bit of a lull in the wet weather from tonight to tomorrow evening when the rain would be more intermittent.
After sunset on Wednesday, the rain would bring 50mm of water down for about 12 hours.
The second round of rain could cause problems with flooding and tree damage, he said.
The second bout of weather could have stronger breezes softening root systems and bring down branches.
Once the last system moved across the country tomorrow night, things would start to settle down again and the weekend was set to have clear weather.
Tauranga City Council emergency management manager Paul Baunton had a standard heavy-rain protocol which was to clear litter and debris from about 200 stormwater "hotspots" before each storm event, and again afterwards.
"Our residents have a part to play too. You can do this by making sure external drains on your property are clear of blockages, and if you see leaves building up in the gutters or sumps in your street, please clear them if you can," Mr Baunton said.
"When driving in the wet, remember to watch your speed and increase your following distance."