Southern Cross would be dramatically overhauled under a $300 million plan to cope with an unprecedented surge in patronage that is threatening the station's ability to function.

Barely 10 years after the station formerly known as Spencer Street was rebuilt, Fairfax Media can reveal Southern Cross is now struggling to deal with passenger numbers.

Growth has been so acute the station could soon rival Melbourne Airport as one of the state's busiest transport hubs, with long queues to access overloaded platforms and an overburdened coach terminal.

As a result, the station's owner, AssetCo, a subsidiary of industry superannuation behemoth IFM, has lodged an unsolicited bid proposal believed to be worth about $300 million that includes a new elevated bridge running down the middle of the station to provide extra access to its 16 platforms.