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SYDNEY - A man who tried to open a passenger door on a Qantas plane which was making its descent into Sydney is receiving medical treatment after being released from police custody.

A fellow passenger said the man was clearly agitated and had been trying to stand up and move around the plane for about 10 minutes during the descent.

Staff managed to reseat him, but he stood up and moved towards a middle door in the economy section aboard flight QF2 from London via Bangkok early on Wednesday.

"He grabbed the handle but Qantas staff were able to restrain him," the witness who was about two rows away told AAP.

Qantas staff moved the man to the front section of the plane and the flight made a normal landing, touching down at 6.05am (AEST).

"The incident wasn't in any way threatening, but some children did become scared and upset, and started yelling," the passenger said.

Passengers were asked to remain in their seats for another 20 minutes before being allowed to disembark at Sydney airport.

Australian Federal Police (AFP) met the man at the arrival gate and took him into custody for questioning.

The AFP said he had now been released from custody and would not be charged.

"The AFP can confirm an incident occurred on a flight from London via Bangkok to Sydney on 26 August," a spokeswoman said.

"Members of the AFP attended the arrival gate and escorted an individual from the plane.

"The individual is no longer in AFP custody.

"He is instead under the care of appropriate medical authorities."

A Qantas spokeswoman was unable to confirm how many crew members had to restrain the man but denied he had handled the cabin door.

"A disruptive passenger on Qantas flight two was restrained by cabin crew on descent into Sydney this morning," a Qantas statement said.

"While the passenger was in the vicinity of an aircraft door at one stage during descent, he did not handle the door.