Several other passengers were being treated for injuries, emergency services said. One person was airlifted to a hospital in Melbourne.

Images showed a number of twisted carriages had left the track, with one on its side.

It was unclear what caused the train to derail.

The train was reportedly carrying 160 passengers and five crew members when it left the tracks at about 19:50 local time (08:50 GMT) on Thursday, some 45km (28 miles) north of Melbourne, the Victorian state capital.

It is understood to be the main long-distance service between Melbourne and Sydney.

Passenger Rob Jennings told The Sydney Morning Herald how people were thrown around the carriage for around a minute as the train derailed.

"It just veered off, and all the carriages smashed into one another," he said.

"People were tossing around … there was some screaming - everyone was just grasping on, some in the brace position, preparing for the possibility of something worse."

Passengers also said the driver had announced that he would try to make up time for earlier delays.

Paramedics from Ambulance Victoria said that "a large number of people are being assessed but are not believed to be seriously injured".

They added that four people were taken to hospital "in a stable condition".

Victoria's Country Fire Authority (CFA) said that five carriages, along with the train's engine, had derailed and that helicopters and ambulances were at the scene to assess injuries and transport victims.

All train services that run on the affected line - which connects the stations of Seymour and Shepparton - have been cancelled while an investigation takes place.

Australia's Deputy Prime Minister Michael McCormack posted a statement on Twitter announcing that "a full and thorough investigation" would be carried out by the Australian Transport Safety Bureau and the national rail safety regulator.

"Our thoughts are with all those involved and their loved ones," it added.