The union representing workers on Perth's Forrestfield-Airport Link project has condemned the head contractor over safety concerns, calling it the "worst builder" to have ever operated in Western Australia.

Tension between the Construction, Forestry, Mining and Energy Union (CFMEU) and contractor Salini Impregilo on the $1.86 billion project have intensified after a series of onsite incidents.

The CFMEU claimed in a letter presented to the company on Thursday that the tunnel's construction had been compromised because of a "lack of planning and engineering" to ensure its safe completion.

Union members protested over safety concerns on the project as the letter was delivered. ( ABC News: Eliza Laschon )

"We believe there are very significant irregularities that may well leave the taxpayer with an enormous financial liability for years into the future," the letter said.

"We believe another serious injury or death on this project is only a matter of time."

One tunnel worker suffered a serious head injury after being hit with compressed air hose earlier this year, and another reportedly suffered a minor hand injury in November.

Work on the site also came to a halt in September after a water leak flooded part of the tunnel and created a sinkhole on Dundas Road.

Part of the Forrestfield-Airport link tunnel was left under water due to the leak. ( Facebook: CFMEU WA )

The union has called on the State Government to stop work on the project until an independent safety audit can be carried out.

CFMEU state secretary Mick Buchan says enough is enough. ( ABC News: Eliza Laschon )

"Our message to [Transport Minister Rita Saffioti], to the Government, to Salini, is to wake up to yourselves. This project still has a long way to go," CFMEU state secretary Mick Buchan said.

"We're seeing guys that are injured … there is no debate or consultation on what's the right way to do things moving forward.

"Enough is enough … we're not going to take it. The Government needs to step in."

Safety on project defended

In a statement, project director Richard Graham said the link was being constructed to "international standards" and every step was subjected to quality assurance.

Sorry, this video has expired Borers broke through at the end of the tunnel in July after progress stalled earlier in the year

"WorkSafe has completed more than 35 inspections since construction started and last inspected the sites on November 9. There were no notices raised," Mr Graham said.

"We operate eight open sites and provide union access to relevant areas three to four times every week. Unions have made almost 300 visits to our sites since construction commenced.

"We deal openly with union representatives and have no issue with any safety authority reviewing our practices. Safety is our first priority."

Ms Saffioti said the State Government was doing everything it could to protect worker safety.

Rita Saffioti says she has taken on board the "genuine concerns" about the project. ( ABC News: Eliza Laschon )

"We're facilitating meetings with WorkSafe, we've had WorkSafe visit the site on numerous occasions," she said.

"[We've] increased the amount of surveillance workers there from one to three to make sure there is 24/7 coverage."

'Escalation' of safety concerns: Opposition

Ms Saffioti said she had taken onboard the "genuine concerns".

"In particular, some of the issues happened [outside of] the normal work hours, so of course, we responded by increasing the resources," she said.

"I think we are really making it very clear that we want safety to be the priority."

Shadow Transport Minister Liza Harvey said it was a clear escalation of safety concerns on site.

Liza Harvey says the State Government has got a "problem project" on its hands. ( ABC News: Eliza Laschon )

"It is a pretty extraordinary action for a union that is affiliated with the Labor Party so closely to be taking this kind of action on what is a significant project," she said.

"Clearly there are breaches. We've seen them, the union are aware of them. This is a problem project."

A team of engineers is still assessing the reason for the water leak.