Schools are closed and roads are treacherous after the biggest flood in a decade in Opotiki.

Opotiki mayor John Forbes said about 20 per cent of the town was flooded and the damage to the roads could cost Opotiki District Council $1 million after the equivalent of the average rainfall for the month of February fell overnight.

"The most damage is our roads, that costs us a lot of money.

"A really heavy downpour can cost into the millions, it's always the cost the community has to carry.

"It wouldn't surprise me if the cost is a million bucks in the roading network."

Opotiki had 78mm of rain over the past 24 hours, MetService meteorologist Tom Bell said, about as much as it typically gets for the entire month of February.

But Forbes had heard up to 127mm was measured in the mountains.

Rising waters near Opotiki yesterday. (Photo / Supplied)

The council activated its emergency operations centre and put out portaloos because the town's wastewater system had broken down.

"It was a little smack, the rain stopped just in time," Forbes told the Herald.

"Quite a few farms went under water, but they come back out again fairly soon.

"Normally we pump 4000 litres an hour [through the wastewater system], during flooding we pump up to 40,0000 litres an hour."

Forbes said the last "good flood" was in 2003. Every year the council budgets for flood damage and puts the money aside if it's not used.

Opotiki's motels were full of stranded truck drivers last night after the SH2 gorge road to Gisborne was closed.

Bales of silage float down a river near Opotiki. Photo/supplied

Opotiki District Council reported that the Otara River, which runs through the town, has recorded its highest-ever level.

Today NZ Transport Agency is warning drivers travelling along SH2 and SH35 of surface flooding and slips.

At least five schools including Opotiki College and three early learning centres closed yesterday.

The Ministry of Education warned Bay of Plenty parents to keep in contact with schools and early learning services due to the severe weather.

Waiotahe Valley School is closed today because of flooding.

Bales of silage float down a river near Opotiki. (Photo / Supplied)