KiwiRail's handling of the Aratere ferry and its catalogue of mishaps warrants an inquiry according to New Zealand First.
The Aratere was sent to Singapore for three months of repairs after one of the new propellers that was fitted when the ship was extended in 2011, fell off in Cook Strait.
Refitted with its old, smaller propellors, the ferry returned to service last week, but the refitting has slowed it down, forcing KiwiRail to limit it to two return sailings a day.
New Zealand First leader, Winston Peters, said KiwiRail has mismanaged the ferry service.
"In trying to extend it, [the ship] which they were advised against, trying to fix it up after it broke down, which they were told would happen, and then they got Stena Alegra - that thing was an absolute lemon as well. They've racked up possibly between $60 and 80 million in this debacle."
Mr Peters said the Aratere should be sold.
KiwiRail chief executive Peter Reidy said at a news conference on Thursday the Aratere was running slow and crossings were taking 15 minutes longer.
"It is no surprise to us that it is slower. We are surprised it's 15 minutes slower," he said.
There was no way of foreseeing that the decision to fit smaller propellors on the ferry would slow it down, Mr Reidy said.
Trials in Singapore showed the vessel's speed would not be greatly affected but the Cook Strait tides were having a larger impact than expected, he said.
KiwiRail made the decision to refit the original, smaller propellors on the Aratere, even though they were less efficient, because it needed the ship back in service as it carried 40 percent of Interislander freight.
"The risk-free option was really to replace with the older, reliable propellors.
"We knew they were less efficient but at least we knew they'd do the job. You can't get propellors off the shelf," Mr Reidy said.
The refitted propellors were also more fuel efficient, he said.