The $11 billion Metro Tunnel's independent cost advisers have warned its financial backers the project could face many millions of dollars in extra costs and months of delays due to the COVID-19 pandemic.

The Andrews government's flagship rail project has already blown out by up to $3 billion, but leading quantity surveying and construction cost management consultancy WT Partnership has told the project's financiers to expect new coronavirus-related costs and delays.

Work on the Metro Tunnel is likely to be delayed due to coronavirus. Credit:Justin McManus

“The advice is productivity is down,” the company’s executive director David Stewart said in an exclusive interview with The Age. "A conservative view is the delay would be weeks, but it may be months ... which may result in millions of dollars in delay costs."

Other major transport projects such as the $6.7 billion West Gate Tunnel are not immune from coronavirus delays plaguing the construction industry, due to new safety measures and a slowdown in construction material imports.