Waves batter Interislander ferry on its last run back to Wellington. Video / Supplied
Interislander's Cook Strait ferry service will be reduced to just one passenger ship as it struggles with an increasingly unreliable and ageing fleet.
KiwiRail confirmed the service between the North and South Islands will be reduced during August and September.
While two ships will be operating during this time, theValentine is for freight only meaning just one will be left for passengers.
The disruption is due to the Aratere receiving wet dock work until August 9 and the Kaitaki undergoing dry dock work between August 16 and September 28.
Interislander executive general manager Walter Rushbrook apologised for the disruption.
"There will be reduced vehicle deck capacity compared to normal so customers will need to book in advance and may not get their preferred sailing time. Space remains available for foot passengers.
"We will release booking space on our website as it becomes available. During this period we proactively confirm bookings with our customers to enable us to open up space on our ships."
The problem has been exacerbated by the Kaiarahi ferry being "catastrophically" damaged and in need of European specialist assistance when its gearbox failed last year.
"Bearings and gears were catastrophically damaged during the failure. Further, the forces caused by the failure warped the gearbox casing. In order to get access to the gearbox, we needed to remove some utility services and cut through the deck", ministerial briefings revealed.
KiwiRail's Interislander ferry Aratere on Wellington Harbour. Photo / Mark Mitchell
It was first thought the ferry would be back in service by March, that was then pushed out to the middle of this year, but now there's no firm return date.
Rushbrook said they had planned to have the Kaiarahi back in service before Kaitaki's departure for dry dock work.
"But we were recently informed that the American company manufacturing some of the replacement parts for repairs to its gearbox has experienced delays."
Rushbrook said like many things, the issue was due to supply chains being heavily disrupted and they were still waiting for the spare parts to be manufactured.
"We're elevating this with their (the American company's) senior management on a twice-weekly basis to make sure the parts get delivered as soon as possible."
Rushbrook hoped to announce a re-start date in the near future for the Kaiarahi.
The Valentine was leased following the gearbox failure as a freight-only ship to maintain the vital link between the North and South islands while the Kaiarahi wasn't sailing.
KiwiRail has ordered two new bigger and rail-capable ships, which are due to start entering service in 2025 and eventually replace the existing fleet.
Ia Ara Aotearoa Transporting New Zealand chief executive Nick Leggett said KiwiRail has been very upfront about the scheduled maintenance and the fact it's an unavoidable situation.
But he said the industry remained concerned about the delay to the movement of freight.
"This is yet another supply chain pressure that New Zealand is facing at a time when our economy can least afford it. We've got to maintain our productivity and we've got to grow if we're going to fight the thief that is inflation."
