A 25-year-old man from Dandenong is in custody after he allegedly approached the cockpit on flight MH128 while brandishing an unidentified black object that he said was a bomb.

A group of male passengers detained the man as the flight turned around and returned to Melbourne Airport, where heavily armed police eventually stormed the plane to arrest him.

Robert Macdonald heard a commotion before jumping on the man and restraining him with several other passengers.

He said he wouldn't think twice about doing it again and dismissed claims he was a hero.

The Scotsman – who was visiting family in Melbourne – also criticised police efforts after they took more than an hour to board the plane.

"I heard an air hostess shouting 'this guy's crazy' and there was a commotion in the seat in front of me.

"We pushed the guy onto the floor and cabin crew put cords on him and secured him to the ground."

Mr Macdonald said the man's eye's were glazed over and he looked dazed as passengers and cabin crew pinned him to the floor.

"It was just my instinct to get up and help," he said.

"There were three or four guys on him already. He was really struggling but they were big guys so he had no chance."

"I'd do it again."

Mr Macdonald echoed sentiments from other passengers upset with how long it took police to storm the plane.

"When we got into the airport this guy was already pinned down so we thought it was going to be quick."

"Instead we spent an hour and 10 minutes with a potential bomber and potential bomb onboard. What was the delay?"

Fellow passenger Selena Brown said the worst part of the ordeal was waiting 90 minutes for police to board when they could see them outside.

"We were only 10 minutes into the flight and I could hear a kerfuffle behind me. Next thing, a guy ran up the other side of the plane saying 'I've got bomb,'" she said.

"People were screaming and then he was jumped on. There was so many people it was hard to see. Everybody was really calm after that.

"We were told police would take 10 minutes but it was over an hour. I thought it was really disappointing only because we weren't updated all the time.

"We could see them all outside and they weren't coming on to the plane and we didn't know why."

Ms Brown praised the efforts of passengers who jumped on the man after he started shouting about having a bomb.

"They are pretty big burly guys and they weren't letting him get away saying something like that," she said.

"My heart went into my throat a bit but everyone was really calm. The people sitting behind him with little kids, one lady was really upset.

"The passengers were brilliant. Typical thing for Australians to do, to say 'this isn't going to happen'. It was really good."

Don and Judith Urwin were seated in row 32 of the plane, opposite the man in in question in row 31.

Mrs Urwin said she suspected something was wrong when the man got out of his seat 15 minutes after take-off, as the plane was still ascending .

"I just said 'there's something wrong – he's out of his seat – because the plane is still going up,'" she said.

Mr Urwin said he didn't hear the man yelling threats or see what device he was reportedly wearing on his chest.

"After about 15 minutes he opened up the luggage compartment and proceeded to run up the aisle," he said.

"Apparently he put one of the hostesses in a headlock. [She] was yelling out 'stop him'. The passengers jumped on him and got him down pretty quickly.

"You don't think it can happen in this country but it does."

When the plane landed, heavily armed police boarded and found that the device the man had been carrying was harmless.

Passengers Stan and Pam Young said there was "no communication" as they sat on the tarmac, and questioned why it took authorities so long to board the plane.

"They just left us waiting and wondering. The captain said security would be there in 10 minutes but it took two hours," Mr Young said.

"If there was a bomb on that plane we should have been evacuated straight away but we sat there for an hour and a half.

"If security at the airport couldn't come a bit quicker than that, there's something wrong."

Mrs Young said tactical police told passengers to put their heads down and look away as the man was being taken out.

The couple said they weren't made aware of any potential bomb threat until they were taken off the plane and told by a policeman.

The pair are on their way to a holiday in London and said they haven't slept in 24 hours.

"We've lost a couple of days," Mrs Young said.

Another passenger, former AFL player Andrew Leoncelli, told radio station 3AW he had "never seen anything like" the device the man carried.

"It was the size of a watermelon, it was huge, it was black," he said.

"It had two sort of like antennae stuff coming off it, but it also looked like it had an iPhone jack in it, so it could have been just like a beatbox thing."

Mr Leoncelli said the group of men hog-tied the offender and restrained him on the floor him as the plane turned back to Melbourne. 

"I ended up chatting to the lads who did it [detained the man], and they said they seriously did a number on him and put the hog-ties on him really tight ... basically sat on him with their feet on his back with his face into the carpet for the next 20 minutes while we turned the plane around and landed again," Mr Leoncelli said.

Malaysia's deputy transport minister said the device was some kind of mobile phone charger, while Victoria Police superintendent Tony Langdon said the item was something one would "carry around on a regular basis".

Superintendent Langdon said the man seemed to be suffering from a mental illness and had been acting alone.

Police are not treating the incident as terror-related. Mr Irwin said he and his wife were impressed with police efforts and were not afraid of continuing their travel plans. The pair will now get on another flight to Kuala Lumpur.

"Life's too short," Mr Urwin said.

A late night Malaysia Airlines flight was turned back to Melbourne Airport shortly after take-off following a security incident involving a "disruptive" and "threatening" passenger who tried to enter the plane's cockpit while carrying an unidentified black object, according to the airline and local police.

One fellow passenger described seeing the man behaving erratically and threatening to "blow the plane up" before he was eventually pinned to the floor and subdued by others on the plane. A Malaysian government official later told local media the man had been holding a power bank, or mobile charger, not an explosive device.

Dramatic photographs taken by other passengers on board the flight showed heavily armed security personnel boarding the plane after its return to Melbourne. The Malaysia Airlines flight landed safely around 30 minutes after take-off and the passenger was apprehended by airport security. 

Flight MH128, which left Melbourne for Kuala Lumpur at 11.11 pm, was turned back "after the operating captain was alerted by a cabin crew of a passenger attempting to enter the cockpit," according to a statement from the airline.

In air traffic control audio posted online, a male voice can be heard saying: "We have a passenger trying to enter the cockpit." About three minutes later the same male voice can be heard saying the passenger was "claiming to have an explosive device, tried to enter the cockpit, has been overpowered by passengers."

"However we'd like to land and have the device checked," the voice says.

Victoria Police, who are investigating the incident, said in a statement the man had allegedly threatened the safety of passengers and staff before being subdued.

Former AFL player Andrew Leoncelli was sitting in business class, several seats away from the cockpit, when the incident unfolded. He described seeing a man carrying a large black cylindrical object which looked like speaker, which appeared to have an on/off button and a charging port.