Security fencing is being installed hurriedly at Northcote Wharf on the Waitemata Harbour after youths wrecked an electrically operated passenger pontoon by lowering it into the water.

The two young men, 17 and 16, were caught revisiting the scene of the alleged crime on Tuesday night - but only after having prevented several ferry landings by their afternoon antics.

Ferry operator Fullers was able to resume sailings yesterday after temporary repairs to the pontoon, and the wharf management agency Auckland Regional Transport Network began erecting a security gate which it hopes will be ready for remote-controlled use by tonight.

Network executive Simon Laird said the youths risked electrocution by lowering the 15m hydraulic pontoon into the water, causing it to short-circuit and burn out before using it as a slide.

"They lowered it into the sea to use as a hydroslide, destroying all the electricals," he said.

He expected the youths would be issued with a trespass notice, but was not sure whether they would be prosecuted.

Fullers Group operations manager Chris Douglas said the youths abused a ferry captain who noticed them tampering with the pontoon, and refused to get out of the water to allow any attempt to land passengers.

The ferry company decided to suspend all landings until the pontoon could be repaired, and had to charter buses to transport up to 40 passengers to Northcote after setting them down at Devonport.

Mr Douglas said ferry captains were plagued at this time of year by children and teenagers risking their lives by jumping into the water into the path of vessels in a maritime equivalent of the game "chicken".

Both he and Mr Laird appealed to neighbouring residents to be more vigilant to protect what they said were community facilities, if these were to remain in use.

Mr Laird said the Orakei jetty at the eastern tip of Okahu Bay was another favourite target of vandals, and had to be closed over Christmas for tens of thousands of dollars of repairs.

He said vandals destroyed each of seven security lights along the 250m jetty, which is used by charter boats and for starting the annual Auckland Anniversary Regatta, and set a fire after breaking every window in the tower building at the seaward end.

The jetty was reopened in time for the tower to be used to start the regatta, but the building had since been broken into again, even though it was protected by a security gate.

Youths used ropes to swing off the jetty and climb into the building, defying stepped-up night security patrols.

Closed-circuit television, monitored at the Britomart train station, was planned as an extra security measure both there and at every Auckland wharf and station which did not already have one.

Mr Laird said damage to the jetty was a serious safety issue. The lights were needed in case anybody using it at night for fishing or other activities fell into fast-flowing water below.

A sign at the entrance to the jetty appeals to the public to ensure it remains open by removing all fishing lines and rubbish, and not dumping "offensive waste" off the side.