One of Melbourne's busiest arterials will be widened under a $400 million plan to ease congestion across the city.

Premier Daniel Andrews is expected to announce on Sunday that sections of the Monash Freeway will be increased to 10 lanes to alleviate bottlenecks in the outer south-east.

Under the proposal, the freeway would be widened to have five lanes running each way between EastLink and the South Gippsland Highway, and three lanes each way further south-east to Clyde Road.

The idea of upgrading sections of the Monash freeway gathered pace in recent months, with the state government and tolling company Transurban assessing proposals as part of the business case for the so-called Western Distributor, a $5.5 billion plan to ease traffic over the West Gate Bridge.