A new report has found an Interislander ferry's propeller fell off because it was poorly fitted.
KiwiRail has been criticised for not following expert advice when it installed the propeller while extending the Aratere in 2011.
Two years later, in November 2013, the starboard propeller and a section of the propeller shaft fell off in Cook Strait.
The ferry finished its crossing thanks to a back-up system.
Transport Accident Investigation Commission's (TAIC) chief investigator Tim Burfoot said the propeller was fitted poorly. That was likely because KiwiRail either did not seek, or follow, expert advice.
As a result, he said it wore down parts of the shaft assembly, leading to corrosion.
This caused the propeller shaft to crack, which then grew so large the shaft could not cope with normal operating loads.
KiwiRail's report said there was an irregularity in the propeller. The company insisted it followed the manufacturer's advice.
KiwiRail manager of network services Todd Moyle said the propeller broke because of a manufacturing defect.
"We commissioned it from international experts and they have quality assurance processes around that manufacturing.
"We do support the finding in the TAIC report which indicates that the stand for manufacturing these propellers is reviewed. I think the technical properties of them has progressed a lot in recent years and maybe this stand is timely for a review."
After the properller's failure, the ship was sent to Singapore for repairs, costing millions of dollars, but on its return to service one of its six engines failed.
In September this year, it was out of action because of problems with its portside rudder. KiwiRail said the rudder issue had nothing to do with the propellors.
In October, propulsion problems meant it returned to Wellington.