One of the Bluebridge Cook Strait ferries was forced to return to port after suffering a "fault" as it tried to leave Wellington this morning.

The Strait Feronia ferry departed Wellington about 8.15am but quickly ran into trouble and had to be escorted back to port by tug boats.

In a recent update, it is understood the issue, concerning the control pitch propeller, has been resolved and the ferry was expected to leave about 12.30pm - more than four hours after its scheduled departure.

Ferry crew reportedly told passengers over a loudspeaker this morning there had been a "fault", but gave little information as to the exact issue.

The Herald has approached Bluebridge for comment.

Passenger Shaun Rolston had been sleeping in his cabin when he heard the message.

Heading outside, Rolston said it appeared the ferry's ability to turn had been compromised.

He said the crew attempted to solve the issue while still in the harbour for about 30 minutes until they decided to return to port.

"We couldn't turn, they tried to fix it while we were out there but they decided to come back for health and safety reasons," he said.

"It's been pretty blustery."

The ferry set sail at 8.15am but didn't make it very far when it ran into trouble. Photo / Shaun Rolston

The latest update at 12pm the propeller had been fixed and tested several times, and a departure was expected within the next half an hour.

Rolston said the roughly 200-300 people onboard were largely understanding, despite the significant delay it imposed on their travel.

He also noted ferry staff had been "great".

As someone who travelled across Cook Strait regularly, Rolston said it was the first time he'd had such an experience.

Last month, the Herald reported the Interislander's Cook Strait ferry service would be reduced to just one passenger ship as it struggled with an increasingly unreliable and ageing fleet.

KiwiRail confirmed the service between the North and South Islands will be reduced during August and September.

While two ships would be operating during this time, the Valentine was for freight only - meaning just one will be left for passengers.

The disruption was due to the Aratere receiving wet dock work until August 9 and the Kaitaki undergoing dry dock work between August 16 and September 28.

Interislander executive general manager Walter Rushbrook apologised for the disruption.

"There will be reduced vehicle deck capacity compared to normal so customers will need to book in advance and may not get their preferred sailing time. Space remains available for foot passengers.

"We will release booking space on our website as it becomes available. During this period we proactively confirm bookings with our customers to enable us to open up space on our ships."

The problem has been exacerbated by the Kaiarahi ferry being "catastrophically" damaged and in need of European specialist assistance when its gearbox failed last year.

- Adam Pearse, NZ Herald