An Interislander ferry with more than 600 passengers and crew on baord turned back to Wellington due to a security threat.

Photo: RNZ / Dom Thomas

Police said they received a report at 9.05am about an item of concern on the ship.

The ferry had left Wellington at 9am and about half an hour later there was an announcement on board saying it would be turning back.

RNZ has been told that as a precaution passengers on the Kaitaki were asked to congregate on the top deck at the front of the ship.

The vessel returned to dock in Wellington and police cordoned off the in the Aotea Quay area around the terminal.

Police said passengers were being evacuated while officers undertook enquiries on-board.

Passengers were allowed off the ferry but those with vehicles were asked to surrender their keys to police, and to wait in the terminal.

One person who was about to hand over his car keys said passengers were told there would be at delay of at least two hours, but that people were generally calm about the situation.

Around 11am, one of the passengers said they'd been given clearance to reboard the ship and by midday the ferry had left again for Picton.

Police had were advised to avoid the area while the incident was dealt with.

There were 584 passengers and 84 crew members on board.

Photo: RNZ / Dom Thomas

Photo: RNZ / Dom Thomas