'TEETHING PROBLEMS': The ferry Stena Alegra, which is filling in for the Aratere after it lost its propeller on a sailing from Picton to Wellington in November.

A passenger sailing has been cancelled after Interislander's replacement ferry Stena Alegra, broke down in Wellington Harbour this morning, in just its first week of service.

In a situation eerily reminiscent of Aratere, which also broke down in her first weeks of service on the Cook Strait run more than a decade ago, the Stena Alegra broke down about 5am after leaving its Wellington berth for Picton.

Interislander general manager Thomas Davis said the crew discovered a fault soon after the ship left the terminal on a freight-only sailing.

The ship had to anchor in the middle of the harbour after one engine failed, affecting one propeller's control systems.

Stena Alegra's scheduled return passenger sailing from Picton to Wellington, due to leave at 9.05am with 148 vehicles booked, was cancelled. Interislander would re-book passengers and cars on different sailings.

"Our staff are working hard to find a solution to the fault and get the ship back into operation," Davis said.

"We apologise to our customers for the disruption and will issue an update as we have more news."

Davis said Maritime NZ would be notified about the problem as a matter of routine.

The Stena Alegra was to be brought back into port after the Kaitaki left this morning.

There was a light southerly breeze in Wellington Harbour this morning and the weather was fine and calm.

The Stena Alegra, which is replacing the crippled Aratere while it is repaired, made its first passenger crossing on Monday night, but has since been regularly holding up other sailings because of problems with its bow doors and stern ramp.

It sailed two hours late from Wellington on Wednesday morning, and more than half an hour late in the afternoon.

The Kaitaki was forced to wait in the middle of Wellington Harbour on both occasions until it could get into the shared berth at the terminal.

Davis said the delays resulted from "teething problems with equipment, including issues with the raising and lowering of the ramp, some loading issues, and most recently an issue with the bow door".

"We can confirm that some improvements are being made to the bow door equipment by the ship's owner as a warranty issue."

He said none of the issues with its doors and ramp this week posed safety problems, and apologised to Stena Alegra and Kaitaki customers over the delays.

The 21,000 gross tonne Stena Alegra was built in Spain and has been chartered from its British owners as a replacement for the Aratere, which lost its right propeller on a sailing from Picton to Wellington on the night of November 5.

The Stena Alegra previously operated between Poland and Sweden, and ran aground during a severe storm in October.