Rough sea conditions have caused more delays to Interislander's troubled Stena Alegra ferry.

The replacement ferry, which broke down on Friday after being in action for less than a week, had its 5pm crossing cancelled yesterday due to 4m swells.

"Passengers were given the option of either taking the 7pm sailing on another ferry, the Kaitaki, or crossing on the ferry this morning,'' a KiwiRail spokeswoman said.

"The Kaitaki is a ferry we've had for a very long time so obviously our crews are very comfortable and very familiar with how she handles in those kinds of seas.''

The Stena Alegra is due to sail at its next scheduled crossing at 8pm tonight, the spokeswoman said.

The ferry, brought in as an interim replacement for the Aratere after the vessel lost a propeller on an interisland trip in November, suffered a partial loss of power on its departure from Wellington on Friday morning.

The power loss affected some of the control systems for one of the propellers. It was cleared to sail later that day after being checked by staff.

- APNZ