Days after its maiden voyage, one of the new Spirits has been escorted by a tugboat to Geelong as the Australian Transport Safety Bureau investigates a recent "steering issue" on the other new Spirit.
Spirit of Tasmania IV has arrived in Geelong four hours late after suffering from a steering issue and requiring a tugboat escort through Port Phillip Bay.
Ferry operator TT-Line said a "fault" was identified "as the vessel was preparing to transit through Port Phillip Heads".
It comes as transport safety authorities investigate a "steering issue" on the second of the new ferries, Spirit V, during a trial voyage two weeks ago.
One of the new Spirit of Tasmania ferries, Spirit IV, has arrived in Geelong four hours late after identifying a "fault" and requiring a tugboat escort into Port Phillip Bay.
The ABC understands the fault relates to a steering issue involving the ship's autopilot.
A similar autopilot steering issue affecting TT-Line's other new ferry, Spirit V, is currently under investigation by the Australian Transport Safety Bureau (ATSB) after an incident late last month.
Spirit IV, which embarked on its first passenger voyage with much fanfare on Saturday, left Devonport last night and was due to dock at Geelong at 6am.
However, passengers were told early in the morning that the ferry would be arriving late due to an undisclosed reason.
"The person came on the PA and announced the arrival would be delayed till about 8 o'clock [this morning]," passenger Mitchell Bruce, who runs the Ferries of Tasmania Facebook group, said.
The ferry ended up docking at Geelong about 10:20am.
A track map shows Spirit IV made several turns outside Point Lonsdale before entering Port Phillip Bay.
A tugboat shadowed the ferry as it made its way to the terminal.
In a statement, TT-Line said: "The Master of Spirit of Tasmania identified a fault as the vessel was preparing to transit through Port Phillip Heads on Thursday morning."
TT-Line chief executive Chris Carbone said tug support was deemed appropriate.
"The tug had to travel to the ship from Geelong, hence the delay," he said.
The company said it was "investigating the matter and has engaged technicians to diagnose the issue".
"We apologise for the inconvenience to our passengers," Mr Carbone said.
A TT-Line spokesperson said the Australian Maritime Safety Authority will be advised about the incident, as is standard practice.
Today's steering incident is similar to one affecting Spirit V, which is being investigated by the ATSB.
Spirit V was making a trial voyage on October 26 off Point Henry, in Port Phillip Bay, when it suffered a steering problem.
At the time, 94 crew and 20 passengers were onboard, as well as a cargo of freight vehicles.
"At about 2030 [8:30pm], the ship was being controlled using its autopilot when it reached the junction between the Hopetoun Channel and Wilson Spit Channel to the north-east of Point Henry," the ATSB said.
The ATSB said after adjusting the settings, the ship's "rate of turn" increased, with the autopilot subsequently "unable to arrest the turn".
"Steering control was then switched to manual, and port rudder applied to stop the turn," it said.
"Subsequent attempts to control the ship's heading resulted in increasing heading oscillations from port to starboard, resulting in the ship briefly moving outside the charted shipping channel's southern and northern limits.
"In order to regain control of the ship's movement, it was then brought to a stop in the Wilson Spit passing channel to the south of the shipping channel's limit before resuming its outbound voyage."
A final report on the incident is expected to be published in the second quarter of next year.
Spirit IV and its sister ship Spirit V have now replaced the two older ships in TT-Line's fleet.
One of those older ships, Spirit I, suffered a navigational issue as it was about to enter Port Phillip Bay last month, triggering an internal investigation.
The Tasmanian government's ferry replacement program has been plagued with cost overruns and infrastructure delays.
A parliamentary inquiry in May heard the capital program had cost $717 million more than originally expected.
The figure did not include costs associated with berthing both newly completed vessels for many months in Scotland and Victoria while their Devonport facility was still being constructed.
TT-Line is currently trying to sell the two older ferries.