Australia's militant construction and maritime unions are being sued for potentially more than $100 million over an alleged conspiracy to form an illegal waterfront picket line that blockaded a major container terminal at the Port of Melbourne for more than two weeks.

The Victorian International Container Terminal (VICT), which faced the union-led blockade that stranded more than 1000 shipping containers last month, has now launched legal action in the Supreme Court of Victoria.

The company is suing the Maritime Union of Australia and the Construction, Forestry, Mining and Energy Union for damages of $8.2 million in physical damages and up to $100 million for reputational damage and loss of future earnings.

Union members picketed the docks. Credit:AAP

"We're seeking a range of damages, including for transport and supplier costs, shipping line costs, reputational damage and loss of future earnings," VICT chief executive Anders Dommestrup said.