Melbourne airport security breach caused by an innocent 'mistake'

A security breach at Melbourne Airport caused chaos and delays this morning was caused by an innocent "mistake".

All passengers in the Qantas terminal were forced to exit and be-screened just after 5.30am after a person "inadvertently entered the terminal without a security screening", Qantas said in a statement.

Qantas group executive of corporate affairs Andrew McGinnes told 3AW the passenger went through the security gates and later re-entered the terminal, bypassing security.

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"The passenger landed from Perth in Melbourne and they had a connecting flight to Sydney and they went to collect their bags," he told 3AW. 

"The passenger remembered as he was out of the security doors that he didn't need to collect his bag and that it would go through to Sydney so he walked back into the security doors as someone was coming out of them, which triggered an alarm which alerted us.

"We had to pause everything and find the passenger and re-screen everyone."

McGinnes said authorities took a "conservative" approach and said there was nothing more to suggest the passenger did anything but "make a mistake". 

"Unfortunately that resulted in some delays this morning but it was an issue and we had to fix it really quickly," he said. 

"The passenger was found sitting down and having a coffee and he wasn't aware he had done anything wrong."

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Passenger Tim Joyce told Today his flight had been "a minute" from taking off before they were all asked to leave the plane.

"The pilot pretty much told us straight away that there was somebody who slipped through the terminal," he said.

"I think we were basically the last people to get evacuated."

Joyce said despite the inconvenience, the communication from Qantas had been "great".

People were re-screened on a boarding priority basis.

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"I am glad they have evacuated the terminal to make sure everyone is safe, because it is definitely worth a few hours of waiting," Joyce said.

But he admitted there were "a lot of mixed feelings" among passengers in the queue.

Joyce's flight was originally set to take off at about 6am, but was initially postponed to 6.40am, with further delays expected.

"They are still not too sure," he said.

Just hours later, another evacuation took place at Adelaide Airport.

Shortly after 9am ACDT, a security breach forced an "orderly evacuation" of the terminal, the airport said in a statement.

The breach was believed to be linked to the failure of a piece of security equipment.