The future of the passenger train service between Melbourne and Adelaide is up in the air.

The South Australian Opposition claims passengers have been told that Overland bookings have been halted from January next year.

The previous state government provided a $1 million three-year package to Great Southern Rail to help support the service in 2015.

Acting Premier Vickie Chapman said the Government would review the funding model but was not making any guarantees that the train service would continue.

"[There's been] diminishing patronage of trains and the increased patronage to planes and of course car travel has had an impact in relation to that service," Ms Chapman said.

"Probably the best way to deal with that is to have a look at that long term rather than just these annual supplements."

Opposition Leader Peter Malinauskas said more funding was needed to keep the 130-year-old service running.

"Once they stop operating, they rarely come back, so it is critical to keep continuity of this service," Mr Malinauskas said.

"It is an important feeder into The Ghan, which we know is an iconic tourism destination for our state, so this is a service that needs to continue into the future for the tourism sector but also to serve regional communities as well."

The Overland runs twice a week between Adelaide and Melbourne, carrying about 30,000 passengers per year.

Trips take 10.5 hours compared with 1.5 hours flying.

Great Southern Rail has relied on a funding agreement between the South Australian and Victorian governments to fund the service for the past 20 years.

Spokeswoman Melanie Reid said the company was "currently in productive discussions with both governments to renew these agreements".

A Victorian Government spokesperson said Great Southern Rail had agreed to a contract extension until March next year.

"The Government will then work with South Australia and Great Southern Rail on future service models," the spokesperson said.