Four men accused of plotting to bring down a plane planned to use poisonous gas or a crude bomb disguised as a meat mincer, reports said Monday, with Australian officials calling preparations "advanced".
The men -- reportedly two Lebanese-Australian fathers and their sons -- were arrested in raids across Sydney on Saturday evening.
The Sydney Daily Telegraph said they allegedly planned to carry the device on board a commercial flight from Sydney to a Middle East destination as hand luggage.
It said the idea was to use wood scrapings and explosive material inside a piece of kitchen equipment such as a mincing machine.
The Sydney Morning Herald also reported that a mincer was being examined, while The Australian newspaper cited multiple sources as saying it was a "non-traditional" device that could have emitted a toxic sulphur-based gas.
This, it said, would have killed or immobilised everyone on the aircraft.
Prime Minister Malcolm Turnbull said the plans were "advanced" but refused to comment on the conflicting claims over the method of attack.
"I have to respect the integrity of the investigations," he said.
"But I can say that certainly the police will allege they had the intent and were developing the capability.
"There will obviously be more to say over coming days. It will be alleged that this was an Islamist, extremist terrorist motivation."
Australian Federal Police Commissioner Andrew Colvin on Sunday said the aviation industry was potentially a target and that an improvised explosive device was involved.
- Seamless cooperation -
Justice Minister Michael Keenan on Monday called the plans "quite sophisticated".
"It was a plot to bring down an aircraft with the idea of smuggling a device on to it to enable them to do that," he said.
A magistrate late Sunday gave police an additional seven days to detain the men, who have not been officially named, without charge.
Police continued to gather evidence Monday at the five homes raided, warning the investigation would be "very long and protracted".
TV footage on Saturday showed riot police moving on a terraced house in the inner-city suburb of Surry Hills, with a man with a bandage on his head being led away by authorities, draped in a blanket.
A woman at the address denied they had any link to terrorism.
Police reportedly acted after receiving information from an overseas intelligence agency, suggesting the men may have been directed by someone else.
Turnbull would not confirm this, but said "nowhere is far away from anywhere else these days".
"In an age of the internet and the age of social media and the age of instant messaging applications, Syria is not a long way away from Sydney," he said.
"And so that's the criticality of it -- seamless cooperation."
Security has been strengthened at major domestic and international airports across Australia since the raids, with passengers asked to arrive early and to limit their baggage.
This prompted long queues at Sydney's domestic terminal on Monday morning, with officials saying extra checks were likely to continue for some time.
Australia's national terror alert level was raised on September 2014 amid concerns over attacks by individuals inspired by organisations such as the Islamic State group.
Canberra has become so worried that it announced the creation of a super ministry this month combining its security agencies including the domestic spy service, border force and national police to better tackle terrorism.
A total of 12 attacks, before the latest one, have been prevented in the past few years, while 70 people have been charged.
Several terror attacks have taken place in Australia in recent years, including a Sydney cafe siege in 2014 that saw two hostages killed.