The inter-island ferry Aratere is making its first sailing carrying passengers since crossings were cancelled two weeks ago due to engine problems.
The 13-year-old ship has been beset by engine and mechanical problems since returning from a five-month, $53 million refit in Singapore during which the ship was lengthened and given a new bow.
All Cook Strait crossings by the vessel were cancelled after its engines failed outside Tory Channel on the night of 31 October.
It resumed freight-only sailings on 4 November.
KiwiRail's chief executive Jim Quinn says the power management system did not work properly with some of the engines, and some transducers did not work as they should have.
He says the company will now look into who is accountable for the repair costs.
Trials on Friday confirmed the stability of the engines and propulsion systems, clearing the way for the Aratere to resume full services.
The first passenger sailing departed from Wellington just after 2 pm on Monday.