A major expansion of Melbourne's port due for completion late next year is tipped to funnel more than 6000 trucks a day onto the West Gate Freeway, prompting a truck industry warning that congestion on the West Gate Bridge will become dramatically worse.

The looming truck "nightmare" could have been avoided if a dormant freight rail line that runs to the docks had been reactivated instead of being sold to property developers, who have begun building luxury apartments on the site, the head of Victoria's transport industry lobby said.

A truck 'nightmare' looms on the West Gate.

Webb Dock is being expanded at a cost of $1.2 billion to create capacity for an extra one million shipping containers a year in the Port of Melbourne. The dock, directly south of the West Gate Bridge, is due to start operating in December 2016, but will have no rail access.

The West Gate and Monash freeways will provide virtually the only link to and from Webb Dock, which is forecast to generate more than 10,000 daily trips within 10 years, putting further strain on a freeway that already struggles to move more than 170,000 vehicles a day.