KiwiRail says an electrical problem was to blame for the breakdown of a newly-refurbished Cook Strait ferry.
The Awatere had to be towed back to port following engine problems in Wellington harbour about 11.30am on Thursday.
In April this year, 30 metres was added to the length of the ferry, its bow was reshaped and the interior refurbished in Singapore. The total cost was $53 million.
However, the Awatere's return to service was delayed when it was discovered it needed a new power management system. That problem and other unexpected costs put it about $11 million over budget.
Interislander general manager Thomas Davis says Thursday's problem was with the electronics which supply power to the motors for the port shaft.
Mr Davis says it was fixed in a couple of hours, but the ferry had to be towed due to strong southerly winds.
About 150 passengers were affected by the breakdown. The Awatere is back in service.