Bluebridge's Connemara ferry is being monitored more closely after it lost power while sailing in September.
The ferry drifted for two hours after it lost power and blacked out on 19 September while heading into the Cook Strait.
Wellington's harbourmaster Grant Nalder told RNZ there were "early indications" from Bluebridge owner StraitNZ that fuel played a part.
That was supported 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."
Investigation underway
The Connemara ferry is flagged to the Bahamas, which means unlike the Interislander ferries and the Strait Feronia, Maritime NZ does not approve and certify the operation and its crew, carry out the regular flag state control inspections, nor undertake the same audit of its safety management systems.
The Bahamas Maritime Authority (BMA) confirmed to RNZ it had an inspector onboard following the blackout and The BMA was awaiting his report.
"As a result of the incident, the vessel has been placed on The BMA's Enhanced Monitoring Programme so she will be inspected again within one month and then at three-monthly intervals until the Authority is satisfied that the measures recommended to resolve this matter are being maintained."
New Zealand can, and does, investigate incidents relating to foreign flagged vessels, if they occur within the country's territorial waters (12 nautical miles), Maritime NZ said.
"We can look at compliance with both maritime regulations and also, as the Connemara has New Zealand crew, compliance with the Health and Safety at Work Act," it said.
"Maritime NZ's investigation into the Connermara's loss of power is ongoing. It is expected to take several months. We will engage with the Bahamas Maritime Authority as part of our work," it said.
It said the BMA could also launch an investigation.
A stream of woes for the Connemara
The Connemara's woes have continued with the ship losing power again [
https://www.rnz.co.nz/news/national/530515/bluebridge-ferry-connemara-berths-after-losing-power on Friday] in Wellington Habour.
435 people were on board when one of its two engines failed, but deputy harbourmaster Patrick Atwood told RNZ the problem was unrelated to fuel while Bluebridge said it was a minor operational incident.
Meanwhile, on Sunday a return freight sailing on the Connemara was cancelled due to engineering checks in port taking longer than expected.
Maritime Union spokesperson Victor Billot told RNZ mechanical failures on both Bluebridge and Interislander ferries were happening too frequently.
He said although Bluebridge was privately owned, ongoing problems with both operators proved the government needed to invest in new Interislander ferries.
Earlier this month, the Connemara also hit and damaged fenders on Waitohi Wharf in Picton while berthing.