Miraculously, the plot was abandoned at the final stages – at an Etihad Airways check-in counter – on July 15, possibly because the bag was too heavy, police said. 

Further details about the alleged terrorism conspiracy, which resulted in the arrest of four men in Sydney last weekend, have been revealed, including a failed attempt to get the bomb onto an international flight.

Fairfax Media reported on Thursday how members of the alleged terrorist cell took an improvised explosive device, concealed in luggage to Sydney Airport's international terminal. 

However, it was not checked in after the passenger, booked under his own name, was queried about the bag's weight. 

Australian Federal Police Deputy Commissioner Michael Phelan said the passenger did not know the bomb was in his bag. 

"We will be alleging the person who was to carry the IED on the plane had no idea they were going to be carrying an IED," he told reporters in Sydney on Friday. 

With the passenger was his brother Khaled Mahmoud Khayat, 49, who on Thursday was charged with two counts of acting in preparation for or planning a terrorist act.

Police allege he was directing the activities of the alleged plot.

Police allege Khaled Khayat took the luggage with him when he left Sydney Airport while his brother boarded the Etihad flight. He has not returned to Australia.

"We believe he had no idea," Mr Phelan said. 

Khaled's other brother, Mahmoud Khayat, 32, was also charged with acting in preparation for a terrorist act.

The pair have been in custody since their arrests in dramatic raids in Surry Hills, Punchbowl, Lakemba, Wiley Park and Bankstown last Saturday evening. 

The pair's case was briefly mentioned in Parramatta Local Court on Friday. There was no application for bail and the case was adjourned to November 14. 

Police did not find out about the chilling close call until 11 days later. British and US intelligence agencies tipped off their Australian counterparts about the mass casualty plot.

It is understood the overseas agencies intercepted communication between the Sydney men and Islamic State operatives. 

Mr Phelan would not be drawn on whether the failure of the passenger to check in the baggage on a pre-booked international flight raised any warning signs with airline staff.

"I'm not saying it didn't raise the red flag," he said.

"What I am saying is the airline staff are part of the security layers, the multiple layers and everything from intelligence all the way through to physical searching of things.

"The level of training they have, as far as I'm aware ... they are trained to certain standards to look for certain indicators like everybody else. 

"Whether or not those indicators helped bring this about and not getting on the plane, I can't say."

Exactly why the bomb did not make it past check-in is not certain but police are investigating whether the bag was deemed too heavy or Khaled Khayat simply changed his mind. 

The IED, concealed inside a meat mincer, was taken from the airport and partially dismantled. Mr Phelan said police found that device during raids over the past week.

"The IED that was meant to be planted on [July] 15th, yes we are confident, very confident we have found every single component of that IED," he said. 

The cell then allegedly focused on a plan to create a "chemical dispersion device" that would emit a poisonous gas, possibly in an open, crowded space. 

The plot was being aided by a senior Syrian-based IS officer, who had sent the device components via Air Cargo to Sydney. 

Mr Phelan said one of the Khayat brothers charged in Sydney was put in contact with an IS "controller" via one of his brothers.

"One of the two people that has been charged was put in contact with the IS controller around about April this year," Mr Phelan said. 

"It will be alleged he was introduced by his brother who was a member of IS."

All four men arrested as part of the terrorism investigation in Sydney are related by marriage. 

Abdul Merhi, 50, was released from police custody without charge on Tuesday night.

His brother Khaled Merhi remains in custody but has not been charged. 

Those men are related to Ahmed Merhi, who was once viewed as an active recruiter for IS in Syria, where he has been based since 2014

The Age