The Maritime Union says international shipping lines may stop calling at New Zealand ports, unless there's more investment in facilities there.
It says shipping lines could make an Australian port the transport hub for all of New Zealand's goods, making importing and exporting here more expensive.
The union says the Government's hands-off approach to the port sector could harm the nation's transport infrastructure.
But Transport Minister Steven Joyce says neither shipping lines nor exporters are worried about the ports.
He says the Government wants to let the evolution of the port sector continue, avoiding making changes that would not occur naturally over time.