Eight youths were caught on security cameras on Sunday cutting through a fence at Wanganui Port and releasing the ropes that secured a 20-metre barge to the wharf.
Port manager John Blaikie said the barge swung out into the channel but did not leave the wharf. It was returned to its position and securely tied up.
"While we expect the odd release of ropes, the magnitude of this event was concerning," he said, adding that he would like to see a police presence over the summer months.