Bluebridge ferry the Connemara berthing at Picton wharf, helped by a tug, after earlier hitting the wharf while attempting to berth. Photo: RNZ/Anthony Phelps
A problem with fuel appears to be the cause of a Bluebridge ferry losing power in the Cook Strait last month.
The Connemara blacked out after leaving Wellington and drifted for more than two hours off the south coast before being rescued by tug boats.
Wellington's harbourmaster Grant Nalder said there were "early indications" from Bluebridge owner StraitNZ that fuel played a part.
That has been backed up by the Ministry of Business, Innovation and Employment, which said StraitNZ has advised "that they believe fuel issues may have been a contributing factor in the Connemara losing power."
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Both Nalder and MBIE said it was under investigation.
That investigation is being run by Maritime NZ, and it is still ongoing. It could take a year, the regulator said.
StraitNZ had until Monday declined to comment on what caused the blackout, citing the ongoing investigation. A spokesperson told RNZ it would provide a statement on Monday, but did not respond by deadline.
But Nalder said StraitNZ knew what the problem was, and had resolved it - otherwise the Connemara would not be sailing.
"They're not going to steam round for a year wondering why the engine stopped, they're going to sort that out a lot sooner than that," he said.
"They need to know pretty much straight away, why was this a problem and how do we fix it."
Nalder said as far as he knew there was not a problem with the supplied fuel - rather, it was "something that happened when the fuel got to them" - how the fuel interacted with machinery on board.
"Nothing's been indicated to me that there's a broader problem, and I suspect if they thought it was a problem in what was supplied to them, there would have been a lot more focus on that."
KiwiRail has confirmed the Interislander has not had any issues with fuel on its vessels.
The Connemara has been plagued with problems in the last month.
It again lost power in Wellington harbour on Friday night, though deputy harbourmaster Patrick Atwood told RNZ that was a different problem, and nothing to do with fuel.
Bluebridge said there was "a minor operational incident that delayed berthing."
And on Sunday night, a return freight sailing on the Connemara between Wellington and Picton was cancelled due to engineering checks in port taking longer than expected. Nalder said the Connemara should be sailing again on Monday evening.
It was the latest in a series of disruptions, delays and issues with Cook Strait ferries.