The "disruptive" passenger who forced Malaysia Airlines Flight MH128 to turn back to Melbourne was released from a psychiatric facility hours before he boarded the flight yesterday, Victoria police chief Grahan Ashton said.

The Sri Lankan national, aged 25, had allegedly threatened to blow up the plane and tried to enter the cockpit, but the device he claimed to be an explosive turned out to be a powerbank.

Ashton said police believe the man was a "voluntary patient" at the Monash Medical Centre in Clayton, Melbourne, the Sydney Morning Herald reported.

"We are no longer treating it as a counter-terrorism incident," he is quoted as saying.

"We did initially, because of the reports we had and the assessments that were being made. We subsequently, on doing this assessment, determined this was a case involving a mental health issue."

Superintendant Tony Langdon from the Victoria police the man is believed to have a history of mental illness.

He holds an Australian dual citizenship, and is based in Dandenong in the south-east of Melbourne.

MH128 departed Melbourne Airport at 11.11pm and was to arrive Kuala Lumpur at 5.28am today, June 1, but made a turn back to Melbourne after the operating captain was alerted by a cabin crew of a passenger attempting to enter the cockpit, Malaysia Airlines said yesterday.

Deputy Transport Minister Abdul Aziz Kaprawi said yesterday the man was believed to be drunk.

The incident caused several flights departing from Melbourne Airport to be diverted.

Watermelon-sized electronic device

Australian Football League player Andrew Leoncelli, who was on the plane, said the man was carrying a device "the size of a watermelon" which had "antenna stuff coming off it".

"It also looked like it had an iPhone jack in it, so it could have been just like a beatbox thing," he told Australian radio station 3AW.

Leoncelli said the man ran to the back of the plane after the was confronted over the device.

Passengers then pounced on the man, held him down on the floor and hog-tied him.

Another caller to the radio, who claimed his brother-in-law was on the flight said the man had a device strapped to his chest and demanded to see the pilot.

"I think it was a transistor radio, gaffer taped to his chest."

Passengers said heavily-armed police personnel wearing camouflage gear and helmets entered the plane about 90 minutes after it landed and arrested the man. Passengers were not allowed to leave the plane before that.

Passengers stuck at airport

Passengers from the flight were still not allowed to leave the airport at 8am local time - some eight hours after the plane landed, the Sydney Morning Herald reported.

"Passengers have been sent to local hotels in Melbourne and will be departing to Kuala Lumpur throughout June 1," Malaysia Airlines said in a statement today.

Those with critical onward connections have been allocated seats on flights of other carriers.

The authorities have screened all baggage and they are being transferred to the respective flights, it said.