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, the airline and local police say.

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 aircraft landed safely about 30 minutes after take-off and the passenger was apprehended by airport security.

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