'Extremely rare event' caused interislander ferry Aratere's malfunction

The Aratere ferry lost a propeller during a Cook Strait crossing on November 5, 2013.

An Interislander ferry had a propeller malfunction in "an extremely rare event", the boat's operator said.

The Aratere ferry lost a propeller during a Cook Strait crossing on November 5, 2013. The ferry was knocked out of action for seven weeks and KiwiRail had to find a replacement.

Today, KiwiRail said an internal investigation ruled out external factors or a power surge as the cause of the incident.

The most likely cause was an irregularity in the propeller itself, KiwiRail said today.

"In an extremely rare event, the starboard propeller tail-shaft fractured when Aratere was sailing from Picton to Wellington ... The ship continued safely on to Wellington and the propeller was subsequently recovered from the sea floor near Tory Channel," the company said.

The propeller would cost about $200,000 to replace, KiwiRail said at the time.

"Our investigation was led by the international engineering consultancy firm Aurecon and was lengthy and exhaustive," said KiwiRail's Todd Moyle

The propeller would cost around $200,000 to replace, KiwiRail said at the time.

"Since returning to service, Aratere, which is Interislander's only specialised rail-freight ferry, has been a workhorse of the fleet," Mr Moyle said.

He said the ferry frequently made six journeys daily across Cook Strait.

The Transport Accident Investigation Commission was also investigating the incident.