On Monday, KiwiRail released a summary of a report on the breakdown-plagued ferries by international maritime assessors Den Norske Veritas.
Multiple systems on the three ferries were close to obsolescent, including one ship's bridge radar, but they were "operationally safe", just difficult to repair, KiwiRail said in a statement today.
The DNV assessors concluded it was possible to keep the three ships running until at least 2029, it said.
To do so, however, KiwiRail said it would need to spend $7 million more on maintenance each year than it does currently - taking the sum to $36m, up from $29m - and it would need its board's approval for that.
This maintenance would also mean the three ships would have to be out of service for longer each year, it added.
"An enhanced inspection and monitoring regime is already in place across other ships systems that are facing obsolescence."
The Aratere ran aground on 21 June just north of Picton after crew inadvertently hit a button on the autopilot.
It got the best overall rating of the three: "good", as opposed to "good to fair" for the Kaiarahi and Kaitaki ferries.
Near-obsolescent systems listed in the DNV report included bridge systems, communications systems, and engine control and monitoring system on the Kaiarahi; the bridge radar on the Kaitaki; and the propulsion management, bridge and stabiliser systems on the Aratere.
"DNV commented that although the machinery had been well-maintained, many systems were old and increasingly obsolete - which means it is difficult to get parts for repair - and some system replacement would be needed to ensure reliability," said the KiwiRail summary.
Since the government balked at funding a huge rise in costs for new terminals late last year, speculation had swirled over how KiwiRail would keep the crucial Cook Strait link running.
Den Norske Veritas (DNV) was commissioned by KiwiRail when its grand ferry replacement scheme fell apart.
"DNV reported back this month and has found the existing fleet to be in a better state than thought previously and that it will be able to continue running reliably beyond 2026 with additional maintenance," KiwiRail said in a statement on Monday.
"The steel hulls are in particularly good condition. This is due to Interislander's approach of not sailing in very heavy seas.
"We will need new ferries, but the assessor's findings mean that we can continue delivering reliable services for passengers and our freight customers until new ferries are built and arrive."
There is no sign of when that might be, however.
Two large, rail-friendly ferries were being built in South Korea for half a billion dollars, but that contract had been cancelled.
Recently, the Aratere's steering system and Kaiarahi's propulsion control and monitoring propulsion system were upgraded.
"We are working through what system upgrades will be required in the coming years, some of which are dependent on the government's decision about replacement ferries and when these are expected to arrive."
Maintenance improvements since the Kaitaki lost power in Cook Strait in early 2023, include setting up a dedicated technical team, and doing more checks.
