The Aratere ferry is taking 15 minutes longer to cross the Cook Strait after it was fitted with smaller propellors, and KiwiRail admits it is surprised how much longer it is taking.
The Interislander ferry was sent to Singapore earlier this year for three months of repairs after a propeller - fitted when the ferry was extended 30 metres in in 2011 - fell off in Cook Strait in November 2013.
A return sailing by the ferry was cancelled on Wednesday, just one day after it went back into full service, because it was too slow.
KiwiRail chief executive Peter Reidy said at a news conference on Thursday the Aratere was running slow because it had been fitted with smaller propellors, 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.
As a result, it would do only two return sailings a day, and the Arahura and the Kaitaki would complete extra sailings to ensure passengers were not affected.
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.