The state government says it is investigating an incident in which tens of thousands of motorists were delayed on Tuesday morning when the Sydney Harbour Tunnel was closed for an overheight truck that was not overheight.

The driver of the truck was abused by scores of irate motorists, and the trucking company received telephone complaints by many more, after the Harbour Tunnel was closed about 7.30am when the tunnel's height sensors were triggered.

It took about half an hour to remove the truck to a holding bay, by which time major delays had spread through Sydney's already notorious peak hour traffic.

But the truck, owned by Two Men And A Truck, a removals company, was not too tall to get through the tunnel. According to its driver, the truck has been driven through the tunnel about 12 times in the past month, and will continue to do so.

The driver, Dwight Abbott, was approaching the Harbour Tunnel from the north when he said he saw a stop sign flash on the last noticeboard before the tunnel.

"I thought there might have been an accident inside the tunnel," said Mr Abbott, who was travelling with two others.

"We just stopped there and waited and waited, and then one of the Harbour Tunnel response team came to the side of the truck ... there were about four to five cars in front of us, he came to my window and said 'your truck set off the sensor'."

Mr Abbott had to reverse the truck to a holding bay, where the truck was measured at 4.125 metres high, easily below the 4.4 metre limit.

According to Mr Abbott, the official who measured the truck told him he did not know what triggered the sensor. "The only thing he came up with was that a bird could have flown through it and set the sensor off," Mr Abbott said.

The tunnel was eventually re-opened just before 8am, by which time traffic had built up through the northern suburbs.

Stuck in the holding bay, Mr Abbott received the taunts of people who blamed him for their slow commute. "I tell you what, mate, there was a lot of abuse," he said. "Every second car that was driving past, people were yelling at us."

Fairfax Media asked Roads Minister Duncan Gay and the Roads and Maritime Services if they thought a faulty sensor had triggered the tunnel closure and, if so, whether they would apologise to motorists.

A spokeswoman for Roads and Maritime Services said: "Around 7.35am yesterday morning a truck approaching the Sydney Harbour Tunnel southbound triggered the overheight sensors.

"The driver stopped before the tunnel entrance," the spokeswoman said.

"Inspectors carried out measurements on site and found the vehicle to be just within the legal height limit. Action will be taken against the driver, who was not licensed. Roads and Maritime Services is investigating yesterday's incident."

Mr Abbott said he was not aware on Tuesday morning that his licence had been suspended; after paying a fine, his licence was re-issued by Tuesday afternoon.

The chief executive of Two Men and a Truck, Catherine Gambrellis, said the company received multiple irate phone calls.

"The amount of people abusing us on the telephone was extreme yesterday," she said.

Mr Gay, who has not respond to a request for comment, has announced tougher penalties for drivers and trucking companies that drive overheight trucks into Sydney tunnels, including license suspensions.

Mr Abbott did not receive a penalty for the size of his truck on Tuesday.

The RMS spokeswoman said height sensors were regularly checked for accuracy.

"Operators close to the 4.3-metre height limit on the Sydney Harbour Tunnel route need to make sure their loads are secure and have no interferences on their roof to avoid triggering the sensors," she said.

"If right on the cusp of the 4.3-metre height limit, consider alternative routes."