UPDATED 4.37PM: People heading home from the Coromandel are being told to expect long delays after an earlier crash forced authorities to close the road.

Police said just before 3pm that there are "long tailbacks'' on State Highway 25A , the Kopu-Hikuai Road, in Coromandel.

"The traffic is backed up many kilometres from the coast to Thames,'' a spokesman said.
"It is expected that the long weekend traffic will take some time to clear.''

Earlier, a truck rolled on the highway at Waitakaruru, near the Firth of Thames.

No one was injured, but the road was closed off for some time and diversions put in place as the truck was removed from the site.

Police expect traffic problems for the next two to three hours, at least, and people are encouraged to drive to the conditions; as rain - some heavy - affects much of the country today.

Matt Dalton, who was heading back to Auckland with his family, passed the crash site shortly after the accident happened.

"When we passed, traffic was free-flowing. It looked like police had arrived [and] one tow truck, so the road hadn't shut yet."

They did not see a driver or ambulance, so assumed one had come and gone already, he said.

A police car at one of the diversions at Rawiri Road. (Photo / NZ Herald)

"We commented - my wife and I - that they're going to shut that road to get the truck out. And then we saw another giant tow truck heading the other way, so they were in the process of getting ready to remove it and shut the road when we went through.

"We were lucky. I was flying out to Sydney tonight and so we decided to come back to Auckland early because of the rain. So it actually had a silver lining - we made it through, otherwise I might've missed the flight.''

The circumstances of the crash are not yet known.

Dalton said it appeared the driver had veered off to the left of the road and ended up in a ditch. "He's actually blocking someone's driveway.''

MetService has also issued a severe thunderstorm watch for the Bay of Plenty and Christchurch/Canterbury Plains, telling people to take care on the roads.

A period of rain and a moderate risk of thunderstorms and localised downpours were possible over parts of the Bay of Plenty this afternoon to early evening, mainly in the east and also coastal areas further west.

Traffic is expected to be busy on #SH25A Coromandel Forest Park at Kopu-Hikuai till 10pm today. For more info: https://t.co/9SF4aIIYjn ^MN pic.twitter.com/USpAsjVsBn

— NZTA Waikato/BoP (@NZTAWaiBoP) October 22, 2017

MetService warned that rainfall - between 20mm to 30mm per hour - may cause surface flooding and slips and therefore hazardous conditions for driving.

A couple heading home to Auckland, from Rotorua, said they had been at a standstill in traffic or travelling about 10km/h since the turn-off to Tauranga (State Highway 2) on State Highway 27.

It took them about half an hour to travel between the turn-off to Tauranga and the turn-off to the Coromandel, a distance of about 4kms.

"Cars were already travelling slow because of the weather. It's pouring down,'' they said.

"Some vehicles were towing boats home after being away for the long weekend, so they were slow because of that too.''

Delays are also predicted via State Highway 1 to Wellsford, southbound.

The NZTA said traffic congestion is forecast to be busy through this area up to about 7.30pm today.

Again, motorists were urged to allow for extra travelling time.