A man had to be rescued by the coastguard on Sunday after he jumped from Auckland's Devonport Ferry into the sea.
The man, 53, sustained a head injury and had to be resuscitated.
His condition is described as serious causing concern.
Acting senior sergeant Kyle Bilbrough says a number of passengers and other people at the ferry terminal were detained while police investigated how the man had received his injuries.
Witnesses told police the man had been a passenger on the ferry and had rushed through an exit door and attempted to jump off the ferry, back onto the wharf, when it was disembarking.
