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Repair work on the Aratere Interisland ferry has uncovered cracked rudder shafts.
The vessel has been out of action since last November when it lost a propeller during a Cook Strait crossing.
Repairs to the ship have so far included installing new shafts, old propellers and stabilisers, in preparation for sea trials, KiwiRail said.
"When reinstalling the rudders this week, ongoing checks identified a crack in the ship's port and starboard 'rudder stock' - that is, the shafts which turn the rudders."
Interislander technical staff and the classification society Det Norske Veritas were assessing options for repairs, KiwiRail said.
Any necessary work would be undertaken in Singapore.
The replacement ship Stena Alegra would continue to operate in the meantime, KiwiRail said.