The blockade of the Victoria International Container Terminal (VICT) has been preventing trucks from accessing cargo on the waterfront, and has left stranded millions of dollars of products stored in more than 1000 shipping containers, including Christmas retail goods, fresh food and medicine.

The impact of the rally spread throughout the Port of Melbourne on Friday, forcing costly shutdowns of other terminals.

The nation's largest port operator, DP World, said its entire Melbourne workforce had walked off the job to attend the rally.

It is the latest escalation of aggressive campaigning accusing stevedore VICT of "union-busting" after it stopped giving casual shifts to a union representative.

The company said it stopped giving him shifts after an audit found he was ineligible for security clearance needed to work at the waterfront due to a criminal conviction.

The union claims he was singled out because he had been leading a push among co-workers to unionise and lift their pay and conditions.

Victoria's Supreme Court last week ordered the maritime union to break the costly blockade, but the campaign has since been taken up by supporters in the wider trade union movement, who on Friday vowed not to back down.

"When you look at the array of unions here, I almost feel sorry for VICT," John Setka, state secretary of the Construction, Forestry, Mining and Energy Union, told the rally, attended by several large trade unions.

"You know what? You are in big trouble."

The freight and logistics industry said the effects of the union blockade at the busiest, pre-Christmas time of the year were being felt right across the economy.

"It is an affront to every Port of Melbourne stevedore and freight operator working in and around the port that the Victorian economy is continuing to be held to ransom," Victorian Transport Association chief Peter Anderson said.

"Not only are VICT and the hundreds of freight operators that cannot move containers in and out of the terminal being impacted by this recalcitrant industrial action, so too are hundreds of small business operators and their families that are being denied access to goods demanded by Victorian consumers."

A spokesman for VICT said the stranded imports included EpiPens, frozen prawns and other seafood, toys, Christmas decorations and machinery parts. Stranded exports included frozen meat, chilled cheese, wine, fruit, cotton, clay, timber, lead, zinc and aluminium, hay, grain, wheat and milk products destined for China.

The company has threatened to seek damages of up to $100 million it says it stands to lose because of the ongoing union blockade.

In his expletive-ridden address at the rally, Mr Setka congratulated the dock workers from neighbouring terminals who had illegally walked off the job on Friday morning to attend.

"MUA members that have turned up here today were all threatened with the sack if they came," he said. "You ought to be congratulated for defying these f---ers and coming down here, because it is your fundamental right to withdraw your labour whenever you want."

Friday's multi-union rally comes as the CFMEU and the MUA are pressing ahead with plans to merge into a super union, with the proposal set down for a hearing before the workplace umpire in February.

The Australian Logistics Council said the illegal blockade at Webb Dock was "merely a taste of the industrial mayhem that would be unleashed" if the merger went ahead.

"If the MUA and CFMEU are allowed to merge into a militant mega-union, the sort of wilful illegality we are now witnessing at Webb Dock will spread throughout the nation, and across our supply chains," the council's managing director, Michael Kilgariff, said.

"The actions of the MUA and the CFMEU at Webb Dock are not only an attack on businesses, but on the entire community ... Fresh produce is rotting on the wharf, and merchandise that includes Christmas gifts for children is unable to move."