Most trains on the Warrnambool line ran more than 11 minutes late in December as the service claimed the dubious honour of being the state’s worst performer for 2016.
Public Transport Victoria figures show just 15.4 per cent of services on the line ran on time last month, following a figure of 40 per cent in November and 61.1 per cent in October.
South West Coast MP Roma Britnell said passengers had faced “outrageous” delays for almost two months.
Mrs Britnell criticised Transport Minister Jacinta Allan for failing to come up with a solution and instead focusing on a fourth weekday service that will begin later this month.
“If commuters in Melbourne were dealing with delays like this, heads would roll. Why is it not the same for country passengers?”
Victorian Greens leader Greg Barber called for operator V/Line to give evidence at an inquiry.
"Enough is enough,” Mr Barber said. “The government has been in power for two years and hasn’t fixed these problems.
“I want V/Line to appear before a parliamentary inquiry to explain themselves."
The punctuality goal for every long-distance line is 92 per cent. The Warrnambool line’s average figure for 2016 was 68.3 per cent – the worst in the state by a wide margin.
V/Line spokeswoman Catalina Filip said the main reason for the low punctuality figure was due to temporary speed restrictions.
"The speed restrictions in place are for people's safety – to reduce the risk of incidents at level crossings where it is difficult for drivers to see trains approaching,” Ms Filip said.
“Extra time will be factored into the new timetable later this month, along with 12 extra services each week for passengers."
Ms Filip said the impending upgrade of two level crossings on the line this year would allow some speed restrictions to be lifted.
The restrictions were also introduced in part due to concerns from the Rail, Tram and Bus Union about the poor condition of tracks.
Acting Transport Minister Jaala Pulford also pointed to the speed restrictions, which she said aimed to reduce the risk of accidents at crossings, as the cause of delays.
A new timetable coming into effect later this month will increase some journey times on the line by more than 20 minutes.
Public Transport Users Association of Victoria regional spokesman Paul Wescott described December’s punctuality rate as “extraordinarily bad” and said the poor condition of tracks and the need for duplication of some parts of the line contributed to the problem.