Worksafe inspectors visited the the Forrestfield Airport Link project today after a pipe connector providing fresh air to the tunnel failed.
Nine news reported up to a dozen workers moved to a different area over concerns for their safety and the union was called.
Public Transport Authority spokesman David Hynes confirmed there was an investigation underway.
"Around noon today a pipe connector for a high-pressure air pipe in one of the Forrestfield Airport Link tunnels failed, releasing fresh air into the tunnel," he said.
"The incident took place in the first tunnel just east of the Airport Central station box. There was no explosion and no one was injured.
"The pipe supplies fresh air to the tunnel and the working chamber on TBM Grace. TBM Grace continues to operate using a back-up air compressor.
"The pipe connector had recently been replaced. An investigation is under way into what caused it to fail.
"The PTA is working closely with the contactor, Salini Impregilo-NRW. WorkSafe is aware of the incident."
The incident follows a serious incident at the project on July 7 when 26-year-old New Zealand man Josh Butterworth was injured after being struck by a flailing hose.
Mr Butterworth was in a critical condition and underwent emergency surgery after the hose, which disconnected from a pipe, lashed him in the face.
WorkSafe was investigating that incident as well.
At the time the CFMEU said workers on the project had been told not to talk to the union after the accident and those who had previously raised concerns about safety had faced "draconian treatment".
The $1.86 billion state and federal government-funded project is a joint venture between Salini Impregilo and NRW Holdings and is due to be completed in 2020.
- with AAP