Part of the Acton Tunnel on Parkes Way in Acton has collapsed.
An excavator on the back of a loader entered the tunnel and ripped 60 metres off the top.
No one has been injured.
ACT Fire & Rescue has two pumpers, two Commanders, one Superintendent and the urban search and rescue pod on scene.
The call came through around 10.20am.
There will be traffic delays with the southbound lane of the tunnel blocked.
Update to follow.