Commissioner Mark McCabe said it was too early to tell whether anyone was put a risk when the ACTION buses vented natural gas at the Tuggeranong bus depot last Friday and again at the Woden bus interchange on Wednesday afternoon.

Commuters were evacuated from a bus and several platforms at the Woden bus interchange on Callam Street in Phillip about 5.45pm on Wednesday after gas was released from a cylinder in the roof of the vehicle.

Mr McCabe said the compressed natural gas buses were designed to release gas as a safety measure when problems occurred and ACTION staff and Worksafe ACT inspectors were working together to determine what happened in both instances.

"What is being inspected is what triggered a release at this stage," he said.

He said a technical expert would be called in to help with the investigation.

A spokesman for Territory and Municipal Services said gas had leaked from a bus during refuelling at Tuggeranong bus depot about 6.15 pm on April 12.

He said the Tuggeranong bus depot was not accessible to the public so there was no public risk, and when the leak occurred the staff responsible for refuelling the bus performed an emergency shutdown of the gas refuelling station and followed emergency procedures

"A gas safety valve on the bus activated during the refuelling process," he said.

"The activation of this safety valve results in the controlled venting of natural gas from the tanks on top of the bus where it dissipates into the air."

"The reasons why the gas safety valve activated is under investigation."