The third of the InterIslander passenger ferrries, the Aratere, is preparing to start Cook Strait sailings again after the Kaikoura earthquake a fortnight ago.

The Aratere is due to resume service on Tuesday after the linkspan at Wellington's Ferry Terminal, used to load and discharge vehicles, was damaged in the 7.8-magnitude quake.

"We now have all three of our ferries back on Cook Strait, which is a real boost to our freight customers and to tourists and visitors heading to and from Picton," KiwiRail chief executive Peter Reidy said.

"The Aratere is returning ahead of what we had planned for. We expect rail freight will be able to be loaded next week."

Company operations manager Mark Thompson said Aratere's earlier-than-expected return was the result of repairs to the linkspan being ahead of schedule.

However, the link that allows rail freight to be loaded was still to be fixed.

The Aratere's first sailing, scheduled to leave Wellington at 6am, will be freight-only. The first passenger sailing will be the 5.45pm service from Wellington.