The Kaitaki Interislander ferry docked in Picton (file photo) Photo:
The fault which cut power to the Interislander ferry has been identified as a leak in the cooling system.
The ferry, the Kaitaki, lost power about 5pm on Saturday night in rough conditions, and drifted dangerously close to Red Rocks on Wellington's south coast.
The crew managed to restore power and the ship limped into the harbour, docking about 9pm - seven hours after leaving Picton.
Interislander executive general manager Walter Rushbrook said now it had been identified, the fault was in the process of being fixed.
"The root cause was a fault in the ship's engine cooling system. As is common in ships, there is one water cooling system which is connected to all four of the main ship engines and also to the engines that run the generators for electricity."
A leak in a connection caused a loss of pressure, and sensors had automatically shut down the engines to protect them from overheating.
The connection was now repaired and all the other components of the cooling system had been checked and tested, Rushbrook said.
"The ship will not return to service until Lloyds Register Class Society - an independent third party - has inspected the ship and issued an assurance report to Interislander and Maritime New Zealand," he said.
They hoped to restart sailings on Kaitaki later this week. But the ship would be put through sea trials and carry only freight for a while before returning to normal service.
When bookings become available, updates would be posted on the Interislander Facebook page.
Their contact centre was experiencing high call volumes and long wait times, so where possible, customers were asked to use the website.