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

Police were alerted to the incident about 2.20am and a search is now underway.

“While our enquiries continue to locate the person, we are not seeking anyone else in relation to this matter,” they said in a statement.

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

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

Harbourmaster Grant Nalder could not comment on the incident, 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.”

Nalder understood Coastguard recently launched onto the water to search, while a police boat is not involved in the search yet.

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

Coastguard said two vessels had been deployed and were at the scene of the search.

A rescue helicopter can be seen on Flight Radar circling in the Cook Strait off the coast of Mākara.