A truck carrying an excavator which smashed into the Acton Tunnel did not have a permit to be transporting the earthmoving equipment which ripped 60 metres off the tunnel's ceiling, ACT roads minister Mick Gentleman said.
The incident left traffic on Parkes Way, one of Canberra's major arterial roads, at a standstill and a large section closed in the westbound direction.
Mr Gentleman said the Parkes Way tunnel was "quite high" and it was first time in 40 years that such an incident had happened, noting other large vehicles successfully use the tunnel without the height signage.
"There's a national code for heavy vehicles and tall vehicles and tunnels, such as Acton Tunnel, need to be sign posted if they're below 4.6 metres, the Acton Tunnel is 4.9 [metres]," he said.
"We have a national code ... that requires certain vehicles to have a permit to travel and when that permit is applied the department would go through the regular checks to ascertain the route and ensure that it's a clear route for that sized vehicle.
"Large loads would normally have a pilot and a chase car following ... and we ask them to drive in quiet hours as well.
"I understand at this time there was no permit issued for this particular instance."
He said Roads ACT are working "24/7 to extricate the vehicle and repair the roadworks" but would not be drawn on an estimated time for the road to be clear or whether the government would pursue the earthmoving company for costs.
Mr Gentleman said the government would pay for the repairs "at this time" and would work to ensure Parkes Way was "ready for motorists as soon as possible".
"Those costs will be worked out later on," Mr Gentleman said. "It's something we have to work through, it'll be up to ACT Policing to determine who's at fault."
Under National Heavy Vehicle Regulator requirements, trucks with a combined vehicle and load height exceeding 4.3m need a $70 overheight permit to travel on ACT roads.
The permit can be signed under the name of either an individual or company, but anyone in a "chain of responsibility", ranging from the driver to employers and even the goods loaders, can be liable if road rules are broken.
The maximum court-imposed fine under the national law for "severely" failing to comply with a truck's dimension limit is $10,490.
The employer or primary contractor of the driver, loading manager, loader or packer of anything in the vehicle can be fined an equivalent amount, depending on the circumstances of the breach.
Infringement notices for "substantial" contravention of height limits reach a maximum of $525 for drivers and others in the chain of responsibility.
Sign up for our newsletter to stay up to date.