KiwiRail has completed its investigation into the loss of a propeller from the interisland ferry Aratere two years ago but says the cause remains unproven.
The propeller fell from the ship and plunged to the bottom of Tory Channel.
Recovering the propeller, chartering a replacement ship and sailing the Aratere to Singapore for repairs cost the company millions of dollars.
KiwiRail hints that damages have been paid by the propeller manufacturer - saying that, although the company accepts no liability, KiwiRail nevertheless has confidence about its ongoing commercial relationship with the firm.
The KiwiRail report ruled out all causes of the accident other than an irregularity in the propeller itself.
This could include fretting at the point where the propeller shaft meets the propeller hub.
Other causes could be misshapen or out-of-balance propeller blades, vibration due to misalignment of the parts of the propeller or stress caused by a heavy propeller cone.
A company spokesperson said the manufacturer of the propeller did not agree with its conclusions.
A similar investigation by the Transport Accident Investigation Commission has not yet been released.