An excavator on the back of a low loader ripped 60 metres off the ceiling of the Acton Tunnel on Parkes Way, near the Australian National Botanic Gardens.
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The Emergency Services Agency confirmed no one was injured in the incident, which occurred shortly before 10.20am on Tuesday.
Traffic came to a standstill before emergency services closed the tunnel in both directions under the partially collapsed bridge and put traffic diversions in place.
About 40,000 vehicles travel through the major thoroughfare each weekday, a Territory and Municipal Services spokeswoman said.
ACT Fire and Rescue Superintendent Steve Edwards said it appeared "the load was just too large for the underpass".
The bridge is still structurally intact but is damaged and engineers were on site assessing the structural integrity of the bridge.
"We've got HAZMAT people on scene, police are on scene ... we've got engineers from roads and bridges, WorkSafe, every expert in the ACT who works on this sort of thing is on site," Superintendent Edwards said.
"It hasn't affected the structural integrity of the bridge but the facade underneath has been damaged and power is still off at this stage."
There were fears the crash may have dislodged asbestos, and emergency services enforced a one kilometre exclusion zone down wind, "keeping people well clear and closing roads to traffic in the area."
A number of the tunnel's ceiling panels, made of bonded asbestos, were knocked down by the excavator during the crash.
Initial air quality tests in Acton Tunnel found no asbestos fibres present.
WorkSafe ACT confirmed the absence of asbestos fibres in the air as of 5pm.
The Emergency Services Agency spokesperson said the crash activated the tunnel's sprinkler system, saturating the damaged ceiling panels and eliminating any risk from asbestos fibres in the atmosphere.
"The remaining asbestos risk is in the cleaning up exercise which is currently occurring under the supervision of a licensed asbestos assessor and removalist," the spokesperson said.
"This risk is being managed by the licensed professionals on site. The asbestos assessor has indicated that there is negligible risk to anyone outside of the work area in the tunnel."
ACT Fire and Rescue chief officer Mark Brown said firefighters rendered the scene safe earlier in the day by ensuring there was no one trapped, then evacuating the tunnel.
"At this stage we have a whole range of agencies on scene who are working out the best way extract the machinery from the tunnel and get the tunnel back in action as soon as possible," Mr Brown said.
"It's difficult to say, it will be very hard to remove the machinery safely without removing the underpass of the bridge carriage. To do that safely for the people who are involved it may take some time."
It is unknown how long the Parkes Way lanes will be closed for and motorists are urged to start planning alternate routes home.
People with any health conditions or concerns can seek advice through Access Canberra on 13 22 81.
- with Fleta Page and Emma Kelly
