TRAVELLERS are already reporting delays and “mess” as tougher screening measures come into place in the wake of an alleged terror plot involving a plane.

Prime Minister Malcolm Turnbull warned travellers to expect increased screening procedures at airports around the country following a series of terror raids in Sydney last night.

Four men were arrested after authorities foiled an alleged plot to bring down an aircraft using an improvised device.

Mr Turnbull confirmed the investigation was ongoing.

The government also confirmed it was an Islamic-inspired plot and warned increased screening measures introduced at Sydney Airport last Thursday would now take effect nationwide.

Travellers have been warned to arrive two hours earlier due to “additional scrutiny” while security experts expect the arrangements to remain in place for the foreseeable future, AAP reported.

The ABC reported some passengers at Sydney and Adelaide airports had experienced delays of up to 90 minutes even before passing through security.

ABC journalist Ewan Gilbert said the effects of the tougher measures were already being felt.

Gilbert, who was at the international terminal, told the ABC there was a large police presence and that it was very busy at the check-in desks.

South Australian traveller Jeannine Malcolm was at Adelaide Airport early this morning and also noticed the huge queues there.

That explains why Adelaide airport looked like this at 6:30 this morning. pic.twitter.com/LcU0zfqIWi — Jeannine Malcolm (@Ganjm001) July 30, 2017

News.com.au understands Sydney Airport has not experienced more delays than a regular Sunday.

However some travellers complained things appeared to take longer than usual and they were left waiting around.

I'm at Sydney airport all day bc my flight isn't till 4 and I can't check in until 2 — kinky lea (@LeaMusic99) July 29, 2017

T3 SYDNEY airport a mess. If you are flying come very early. Quest out door at security — jennifer weller (@WellerJennifer) July 30, 2017

Not sure I agree with the 'creation' of a mass gathering 'before' security screening #sydney #airport #terrorplot -closed screening stations pic.twitter.com/oLgfU8Mun5 — Chris Cubbage (@AmlecHouse) July 30, 2017

Another traveller Baz Rao asked why Qantas chose to open only two security gates given the news overnight and the increased security measures.

Mr Rao, who was travelling from Sydney to Melbourne, said there were announcements in the background announcing the new measures.

He told news.com.au there were long lines due to screening stations being closed and it was definitely slower than usual.

Qantas had four lines opened by this afternoon following a busy time during peak period this morning.

Additional lanes will be opened according to demand.

In a statement given to news.com.au a Qantas spokeswoman said both Qantas and Jetstar are working closely with the government and airport partners to implement the increased security measures.

“Given the additional screening requirements, we ask passengers to arrive at the terminal two hours before domestic flights and three hours before international flights,” the spokeswoman said.

“Passengers are also asked to limit the amount of carry-on and checked baggage where possible as this will help to ensure security screening is efficient.”

The airline said Australia has very strong safeguards in place at its airports and these changes are about making them even stronger.

Dr John Coyne, who heads the Australian Strategic Policy Institute’s Border Security Program, said the measure to eliminate threats would give authorities the chance to analyse potential vulnerabilities.

He said passengers might notice more precise x-ray screenings or an increase in swab tests.

He also said there could also be more personnel behind the scenes monitoring abnormal behaviour.

“The question will be how were these guys allegedly planning to get this device onto a plane,” Dr Coyne told AAP.

“Is there an actual vulnerability they were going to exploit, and is it something we need to mitigate.

“That will be the big question that will be faced now and over the coming weeks.”

- with AAP

debra.killalea@news.com.au