Victorian Premier Dan Andrews and minister Jacinta Allan inspect the progress at State Library Station. Credit:Joe Armao

Builders on the Metro Tunnel rail project have reached a milestone at the vast State Library Station site, with a giant "main cavern" for its platforms now excavated.

But Premier Daniel Andrews and his Transport Infrastructure Minister Jacinta Allan continued to bat away questions at the site on Thursday morning about reported cost blowouts and delays to the state's largest ever public transport building project.

The Premier meets with workers at the site of the State Library Station. Credit:Joe Armao

Mr Andrews and Ms Allan held a press conference at the construction site deep below Franklin Street on Thursday morning to showcase the massive building effort, which is using three giant digging machines to excavate 500 tonnes of rock each day.