A Malaysia Airlines flight was grounded in Melbourne last night after a "lunatic" passenger tried to storm the cockpit and threatened to blow up the plane.

The Kuala Lumpur-bound plane was forced to turn back to Melbourne's Tullamarine Airport half an hour after takeoff when a man, who was allegedly drunk or under the influence of drugs, tried to storm the cockpit.

Passengers worked with the flight crew to subdue the man.

A passenger named Andy said he heard the passenger threaten to blow up the plane about 10 minutes after the plane took off.

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"I could hear this idiot saying that he wanted to go in and see the pilot," he told Melbourne radio station 3AW.

"The staff were saying, 'Sit back down, sir, sit back down, sir'.

"He goes, 'No, I'm not going to sit back down - I'm going to blow the plane up'.

"The staff screamed out, 'I need some help, I need some help'. So I jumped up, undid my buckle, and approached him.

"I said, 'Mate, what are you effing doing?' And he said, 'I'm going to blow the f***ing plane up, I'm going to blow the f***ing plane up. And I was going, oh my god, he's f***ing insane.

"So, I took a step back. And I said to my friend, 'Mate this guy's serious. We'd better f***ing do something'.

"So, I approached him and he ran down the side and then he ended up going down the back of the plane.

"And then two lads grabbed him and disarmed him and mashed the crap out of him and put hog ties on him."

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Andy said the man was holding a watermelon-sized "giant black thing". Others passengers described it as an "electronic frequency" device.

"It was huge, it was black and it had two black antennas coming off it, but it also looked like an iPhone jack. So it could have been just a beat box thing," Andy said

"I think the guy was just an idiot."

The wife of another man who was on the plane said passengers and crew helped to tie the aggressive passenger down after he attacked an air hostess.

"He heard a lot of screaming and (an) air hostess calling out for help as a passenger attacked her," Saiqa Chaudhery told news.com.au via Twitter.

"Some other passengers and crew tied the man down and (the) flight landed back at Melbourne Airport 25 minutes after takeoff."

One passenger wrote on Facebook that the man had issued a "midair terror threat".

"The guy was trying to open cockpit and hijack the plane," Melbourne man Dhana Sekaran posted.

"About eight guys, of course including me, jumped and restrained him.

"Thank god I am on board to sort this s*** out. Never thought I will be back to Melbourne this quick."

Lisa Wood, whose father was on the plane, said a "lunatic" announced that he had "a bomb" about 20 minutes into the flight.

"Flight attendants have got fruit cake tied down in between seats," she wrote on Facebook.

The claims about a "terror threat" and a "bomb" have not been confirmed by the police or the airline.

Heavily armed police boarded the plane after it landed in Melbourne and all passengers and crew left the aircraft without injury.

Tullamarine Airport was placed into lockdown and flights were diverted to Avalon, near Geelong, AAP reported.

Aviation reporter Brendan Grainger said some flights resumed earlier Thursday morning, but the north-south runway remained closed.

The airline said the flight, MH128, departed Tullamarine Airport at 11.11pm Wednesday and was scheduled to arrive at the Malaysian capital at 5.28am Thursday.

It confirmed that the "disruptive passenger" tried to enter the cockpit, but stressed "at no point was the aircraft 'hijacked'".

"Following the incident on MH128, the disruptive passenger has been apprehended by airport security," Malaysia Airlines said in a statement.

"Safety and security are of utmost priority to Malaysia Airlines. The airline wishes to apologise for the inconvenience caused.

"Passengers have safely disembarked the aircraft and will be screened by Australian authorities."

The airline has offered affected passengers accommodation at hotels and tickets on the next available flights or on other carriers.

Victoria Police confirmed that the man did not gain entry to the cockpit.

"The man was subdued and a safety plan was enacted," Senior Constable Adam West said in a statement.

"The plane landed safely at the airport and passengers are currently exiting the plane and speaking to investigators.

"There appears to be no imminent threat to passengers, staff or public and the investigation is ongoing."