A person has gone overboard from a ferry in the Cook Strait overnight, police say.

A union representative says incidents like this are not normal and have left crew feeling “weird”.

Police were called about 2.20am and a search is now underway.

Police said the ferry crew realised “some time after” the person went overboard, at which point they notified police.

Kiwirail said the person had gone overboard from the Interislander’s Kaiārahi ferry.

Maritime Union Wellington branch secretary Fiona Mansell said she was only aware of three occasions in her 16 years being involved with Interislander where this had happened.

“This isn’t normal,” she said.

“This is kind of like a rare thing.”

A rescue helicopter can be seen flying in a pattern over the Cook Strait. Image / Flight Radar

Mansell said the feeling on board was “weird” for crew members.

“My members, they’re doing alright. Things like this, it tends to draw them closer together to look after each other.”

Having spoken with crew at 6am this morning, Mansell talked to Interislander about getting counselling on board when the ship arrives back in Wellington Harbour this afternoon.

She was conducting welfare checks with members, however said, “sometimes the shock doesn’t happen till later on, till they come off the vessels”.

Another representative from the union’s head office was also being sent to Wellington Harbour to meet the members when they get in.

Two vessels were at the scene of the search, Coastguard said.

Harbourmaster Grant Nalder could not comment specifically but said situations like this were “usually noted by the ship they’re on, the vessel they’re on. Ideally [the ship] notices so you have a time and place.”

A rescue helicopter can be seen this morning on Flight Radar circling in the Cook Strait off the coast of Mākara, west of Wellington.