An engine part needed to get the Interislander's Aratere ferry back sailing at full steam isn't likely to be installed until next year, leaving only one of the three ships running normally at a busy time of year.
It means the Aratere will continue to run on reduced power, only travel in less severe weather and sail a different course – entering and exiting Queen Charlotte Sound through the northern entrance – during the busy Christmas holiday period.
The ferry has had problems since it get back from Sydney in September, where it underwent scheduled maintenance.
Just a few weeks after the Aratere's return, a broken part of the engine started causing abnormal vibrations onboard.
That part of equipment has now been taken out of service until a replacement arrives from overseas.
Interislander general manager Walter Rushbrook said he's not concerned at the moment that issues with the Aratere will cause delays over the summer period.
"We are just continuing on our normal routine of two returns per day, so unless something adverse happens we will be keeping to that."
Rushbrook said they wanted the ferry to get back to normal as soon as possible.
"We have got a part being manufactured overseas and that will arrive around about Christmas time and we will install it very quickly."
The engine trouble is the latest in a string of issues for the Interislander's fleet of three ships.
The Kaiarahi ferry is out of action until at least March, as it requires major repairs to its gearbox after a sudden failure.
This leaves the Kaitaki as the only boat running without issues.
Because of the strain on services, the Interislander fleet will get an additional ferry - The Valentine - mid this month.
It will be freight only and is likely to arrive just before the part for Aratere, Rushbrook said.
A KiwiRail strike also looks to cause issues for ferry sailings.
KiwiRail workers have voted for a nationwide strike affecting all rail operations, including rail ferries and the Auckland and Wellington Metro systems. The industrial action will take place on December 16 and 17 and affect both the North and South Islands.
The union is pursuing a pay rise of 8 per cent for its KiwiRail employee members.
Rushbrook told the Herald they are working on contingency plans should the strike go ahead.
"Obviously we won't be able to sail so the period leading up to it, and the time after the disruption, we'll just make sure we put on as much capacity as possible and put on extra sailings."
KiwiRail and Korean shipyard Hyundai Mipo Dockyard this year signed a contract for two new state-of-the-art Cook Strait ferries.
The first is due to arrive in 2025 and the second in 2026. The contract price for the two ferries is $551 million.
The two rail-enabled ferries will be able to carry nearly double the number of passengers, and commercial and passenger vehicles compared with the current fleet.