A person has died suddenly on a vessel at South Port in Bluff.

Police were called to the port at 9.30am, and are working with Maritime New Zealand to work out what has happened.

The Maritime Union said it understood a foreign crew member died on a container ship while cranes onboard were being moved.

The union said the ship was believed to be the MSC Resilient III, registered under the Liberian flag.

That made the ship a flag of convenience vessel, which were "notorious for the issues around safety and seafarers' rights", the union said.

The death was a sad and stark reminder that the maritime industry was dangerous, the union said.