Passengers left in dark over security threat on plane at Sydney Airport

Passengers on an Air New Zealand flight that was grounded at Sydney Airport say they had no idea there was a security threat until they saw it on the news.

The incident is at the centre of a major AFP investigation.

The flight from Wellington was isolated at the end of the runway with 154 passengers on board.

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"We were just sitting on the tarmac for an hour," Mercury Lin said, adding it "seemed weird".

Some passengers told Radio New Zealand there had been a bomb threat, and the view outside the window backed that up.

"We weren't feeling scared," Pauline Crown said.

"Ambulance and police outside, that was the intimidating part."

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Dozens of ambulances and fire trucks raced to the scene and heavily armed tactical police took up positions.

There was no cabin announcement.

Some passengers found out they were at the centre of a security scare by watching live news reports on their phones.

After an hour the plane moved closer to the terminal.

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As passengers were allowed to leave, their carry-on bags were given a once over by sniffer dogs.

While it had all the hallmarks of a bomb threat, all Federal Police are saying is they made a thorough search of the aircraft, luggage and passengers, and nothing suspicious was found.

Air New Zealand apologised for the inconvenience, adding the safety and security of passengers and crew was of "utmost priority".