The Maritime Union will challenge the lawfulness of the decision by Ports of Auckland to sack nearly 300 striking workers, but port management is confident it is in the clear.
The union is seeking a ruling from the Employment Court on whether it is against the law for port bosses to dismiss the workers while a collective agreement was being negotiated.
Maritime Union national president Garry Parsloe says port management has failed to bargain in good faith throughout the long-running industrial dispute.
However, the Ports of Auckland is confident of its legal position.
Ports of Auckland chairman Richard Pearson said the company was not worried by the threat of court action.
"We have followed the letter of the law right through and I can assure you that there has been no stone unturned to actually make sure we have covered all legal issues," he told Radio New Zealand.
The port has lost $25 million in revenue as a result of the strike action, Mr Pearson said.
The estimated $9m in redundancy payouts would be paid for by the efficiencies gained as a result of restructuring, he said.
Redundancy talks are expected to get under way this week.
The sacked workers have been invited to apply for jobs with the three stevedoring companies that will supply labour to the port.
The Maritime Union will hold a public rally on Saturday afternoon to drum up support for its position.
Mr Parsloe said the rally, to call for secure jobs and a sustainable port, will be attended by a number of international unionists, including 95 members from the Maritime Union of Australia and union presidents from ports on the west coast of the United States.