A new wave of bomb threats targeting passenger planes across Europe has triggered emergency diversions, fighter jet escorts and airport evacuations, as authorities scramble to identify who is behind them.

Nearly 30 flights were affected on Thursday, with aircraft diverted across several countries and passengers evacuated while specialist teams searched for explosives.

At Sydney Airport on Thursday night, armed Australian Federal Police (AFP) officers met passengers arriving on an All Nippon Airways (ANA) flight from Tokyo.

Patrick Wild was returning from a two-week holiday when ANA flight NH899 touched down in Sydney at about 8pm.

"We were told there was a security issue and that we'd have to stay onboard for another 20 minutes but that ended up being an hour and a half," Mr Wild said.

"We eventually got off the plane and the AFP were waiting for us and told us in pretty bold terms to line up in a single file and drop all our bags on the floor.

"The dog ran up and down to check all our bags. There must have been five or six, at least, AFP guys with semiautomatic rifles."

He said passengers were then taken to an area near the duty-free shops, where they waited for another hour, before being told they could collect their luggage.

"At no point did we get any communication about what was going on," he said.

"It was only when I was reading the news that I realised there had been bomb threats made overseas and that maybe something similar had happened."

In a statement, the AFP confirmed it conducted "precautionary searches after threats were sent to an airport" just after 6pm on Thursday.

"The safety and security of Australian airports, passengers and airport staff is our priority," an AFP spokesperson said.

Officials are yet to confirm whether the Sydney security incident is linked to the wave of threats targeting European aviation.

Is it safe to fly in Europe right now?

There is no evidence that explosives have been found on any of the aircraft searched in the latest wave of threats. All reported incidents have so far been treated as false alarms.

But British aviation security commentator Julian Bray told ABC Radio National that authorities cannot take chances.

"It might be a hoax, but as far as we're concerned, this is absolutely for real," he said.

Britain's Transport Secretary Heidi Alexander described them as "malicious hoax threats".

The European Union has activated an aviation security information-sharing mechanism — essentially a group chat — enabling countries to share information about the threats and whether they are connected.

Which airlines have been targeted?

• Ryanair
• Wizz Air
• Aer Lingus
• Finnair
• LOT Polish Airlines
• easyJet
• Iberia
• Air Europa
• Air Serbia
• Aegean Airlines
• SAS (Scandinavian Airlines)

At least 27 flights were disrupted across Europe on Thursday.

Who is behind the threats?

The identity of those responsible remains unknown.

But Mr Bray said a series of emails sent to aviation-related accounts have been attributed to Russia.

"A series of emails to unmonitored boxes are going out and they're going at two at a time and they have strap lines saying 'bomb threat', and they're supposed to be from the fighters of the Male State," he said.

"And this is a splinter group or a single cause group out of Russia."

Mr Bray said other possibilities should not be ruled out, including involvement by North Korea or China.

"Other people are saying we shouldn't exclude the fact that North Korea and China might be interested in doing this and it might be a diversionary tactic."

There is no confirmed evidence linking Russia, China or North Korea to the threats.

What happens when a plane receives a bomb threat?

A threat can be relayed to air traffic control, which passes the information to the pilot.

The captain assesses the situation and is given options, including diverting or continuing to the destination.

Mr Bray said threats were graded according to their urgency.

"They do grade these calls between extremely urgent, not so, and possibly, well, we'll look at it. But everyone so far has been classified as very urgent indeed," he said.

What happens when the plane lands?

Aircraft are directed to a remote part of the airport, where passengers are evacuated and emergency services are waiting.

Mr Bray said bomb detection dogs were then brought on board, while cabin crew also searched the aircraft using a grid pattern.

The aircraft and luggage must be checked before the plane can be cleared.

Does travel insurance cover a bomb threat?

Not necessarily. Travellers caught up in bomb scares could find their insurance does not cover the cost of missed connections, accommodation or alternative flights.

Most travel insurance policies exclude claims arising from acts or threats of terrorism, which can include disruption caused by a bomb threat, even when no explosives are found.