A person on board the ferry told the Herald the man was part of a "drunken group" of passengers.
Staff had herded passengers back inside following the incident.
"Earlier another member of this drunk group got over the back rail of the ferry and [swung] from a rope. He was reprimanded by the staff but the group continued behaving in an unsafe way," the passenger said.
Police met the ferry at the Auckland terminal.
A Coastguard spokeswoman said a Coastguard Auckland vessel had been sent to the scene but the person was already back on board when they arrived.