The Maritime Union has called out KiwiRail for operating "worn out" Cook Strait ferries that won't be replaced quickly enough with ships that will last for an appropriate length of time.

This follows yesterday's news that Maritime NZ had filed charges with KiwiRail, which operates the ferries, after last year's incident on an Interislander ferry that saw the ship lose power in the strait.

Nearly 900 were on board at the time and a mayday call was made - the ship eventually regained power and was able to make it safely to its destination, but questions have been asked about the suitability of the ferries running the voyages.

Craig Harrison, the Maritime Union's national secretary, said he didn't expect Maritime NZ to go as far as prosecuting the government agency over the matter, but conceded there must be substantial evidence to back up the need for legal action.

"It looks like they've done a thorough investigation in nearly a year and laid charges against KiwiRail around their process and procedures connected with their maintenance management," Harrison told Summer Breakfast.

"I think it's the case of the ships being worn out."

With one Interislander ferry, the Kaitaki, being almost three decades old and the Kaiarahi coming up to 26 years of age, Harrison said both ships were right at the end of their lifespan and in dire need of replacement.

Considering they run a daily voyage between the North and South Island, navigating rough waters and meeting quick turnaround deadlines, Harrison said it was no surprise the ships were struggling now.

Asked how long it would be safe to keep using the ships, Harrison admitted time wasn't a luxury that KiwiRail could afford.

"The plan was to have the vessels gone by the end of 2026 and the two replacement vessels were built to increase the ability to sail in fair weather with more frequency," he said.

"I'd say they expected 25 to 27 years out of those vessels, and now that's been sort of kicked into touch until the Government finds a new solution, but I don't think time is on our side in this case, I'm afraid."

Harrison likened running the ferries to keeping a vintage car on the road - spending large amounts of money just to maintain the vehicle at an unreasonable rate.

"I ask the question of how much time was on hold and additional maintenance to keep these vessels sailing, there's a significant amount of work to keep these things going."

With ships required that will ultimately "last multiple generations", as Harrison put it, he agreed that somebody will need to put their hands in their pockets to fork out money for them at some stage.

The question will be which agency or entity will step in to foot the bill.

"Well nothing gets cheaper when you're building, you only need to look at Transmission Gully, a great project but it cost us a fortune in the end but still a great project."

Harrison said the cheapest time to purchase the vessels needed would be right now.

"The [new] vessels were designed and [are] sitting on the blocks to get built," he said.

"If they want, for instance, three vessels to replace the three vessels they've got now, who's to say it'll cost us three new vessels at a similar size to the two new ones? So I don't think there's an easy way out unfortunately for anyone."