Tunnelling machines on the $6.7 billion West Gate Tunnel could sit idle underground for six more months due to the lack of any plan to deal with large amounts of toxic soil.

The scheme's 300 tunnelling workers will find out early on Tuesday which 150 of them have been retrenched from the Andrews government's flagship project, as the tunnel-boring machines they were hired to operate have not moved a centimetre.

The machines have already sat idle for about five months and the Australian Workers Union said it had been told by the project's managers it would be months before they are operating, casting doubt on whether the project can be delivered by its 2022 deadline.

A West Gate Tunnel construction site in New Street, South Kingsville. Credit:Joe Armao

A key reason for the delays is the discovery of soil contaminated with PFAS – a toxic chemical that led to shutdown of the CFA training college at Fiskville in 2015 – which was discovered when construction began last year.