The Maritime Union is angry Ports of Auckland has advertised dockworkers' jobs in Australia, but the company says the ads appeared by mistake.
The port company is considering outsourcing hundreds of jobs as contract negotiations with the union have stalled.
An advertisement appeared on Monday on the Australian website of recruitment firm Hudson asking for expressions of interest from stevedores and other workers.
The port company says the positions were meant to be advertised in New Zealand and were put on the Australian website by mistake because Hudson is an Australian company.
The union has dismissed the explanation as nonsense, saying New Zealand has enough unemployment without bringing in Australians to take existing jobs.