Leaked documents reveal Victoria's environmental watchdog graded the Metro Tunnel soil's contamination level more than a year before digging started, but is yet to rule on the stalled West Gate Tunnel, six months after boring was due to start.

The tunnel's builders say the lag is a key reason they have threatened to walk away from the $6.7 billion toll road, forcing the Andrews government into an embarrassing impasse on its flagship infrastructure project.

Tunnelling is more than a year behind schedule, with boring machines sitting idle as a legal battle rages over the amount of soil contamination and where the waste should be dumped.

Builders John Holland and CPB Contractors have threatened to enact a force majeure over the digging standstill.

Transurban said in an announcement to the stock market last week that classification "issues" relating to the project's soil were a reason for the builders wanting to leave.