The Aratere ferry will be back sailing tonight, three days after a minor mechanical failure was discovered.

"The Aratere sails Cook Strait 19 hours a day, five days a week," Kiwirail chief executive Peter Reidy said.

"It's a real workhorse for the country, extending SH1 across the sea for our people and our freight.

"In the year to June 30, Aratere missed only two return sailings due to engineering issues out of approximately 900 return sailings."

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Kiwirail's use of two other ferries - the Kaitaki and Kaiarahi, which was brought back into service to replace the Aratere - meant the backlog of passengers and freight following three days of storm last week was now mostly cleared.

Reidy said there was minimal disruption.

"Our engineers worked quickly with the manufacturers and an issue with a seal in the steering system was fixed.

"While the Aratere is often described as a 'lemon', the facts on reliability refute this. Its on-time performance over the past four years, 99 per cent success in scheduled sailings and record passenger numbers each year are second to none.

"Managing that asset is important to Kiwirail, so at the first sign of an issue, we took the decision to thoroughly investigate. At no time was there any risk to the safety of the passengers, crew, or ship."

The first sailing tonight will be at 10.45pm.