The investigation intends to cover whether the navigational issue was a technical fault or human error. (ABC News: Morgan Timms)
In short:
Track maps show Spirit I appeared to go off course and make a wide U-turn near the Victorian coast early Thursday morning.
Ferry operator TT-Line said the issue related to a problem with the ship's navigational equipment.
What's next?
TT-Line is trying to work out if the issue was a technical fault or human error.
TT-Line has launched an investigation after a navigational issue affected one of the Spirit of Tasmania ferries as it approached the Victorian coast last week.
Track maps show Spirit I was heading towards its destination at Geelong in the early hours of Thursday morning when it went off its standard route and made a U-turn close to Point Lonsdale.
This image, from MarineTraffic, shows the Spirit of Tasmania I going off route on the morning of October 15 as it approached Point Lonsdale. (Supplied: MarineTraffic)
"As the vessel was preparing to transit through Port Phillip Heads … the master identified an issue with the ship's navigation equipment," ferry owner TT-Line said in a statement.
"As a precaution, the vessel returned to open water to reset the system before proceeding.
"The reset was successfully completed and the vessel arrived safely at the Port of Geelong, approximately 20 minutes behind schedule."
The track map also includes Spirit I's route over the past week.
TT-Line is owned by the Tasmanian government.
The company's chief executive Chris Carbone said safety was its "number one priority".
"The company is fully investigating the occurrence and has commenced an independent review to better understand the circumstances," Mr Carbone said.
"A report was made to the Australian Maritime Safety Authority (AMSA), which is standard practice for incidents of this nature."
It is not yet known whether the navigational issue was related to a technical fault or a human error.
TT-Line said it elected to have a pilot on board the ship the following evening.
The ABC has contacted AMSA for comment.
Old Spirits to be retired
Spirit I and its sister ship Spirit II have been transporting passengers and cargo across Bass Strait since 2002.
They will make their final sailings from Devonport to Geelong on Thursday and Friday, respectively.
The ships will be replaced by two new ferries, Spirit IV and Spirit V, which will take their first passengers from October 31.
The Tasmanian government's ferry replacement program has been plagued with cost overruns and infrastructure delays over recent years.
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 vessels for many months in Scotland and Victoria while their Devonport facility was constructed.
TT-Line is trying to sell the older ferries, but it remains unclear where they will be berthed until a buyer is found.
Spirit of Tasmania I travels between Devonport in Tasmania and Geelong in Victoria. (ABC News: Morgan Timms)