THE tip-off Australian police received over a conspiracy to bring down a passenger plane came from foreign intelligence sources on the verge of issuing a public travel warning, according to media reports.
It's understood intelligence agencies in the UK and US tipped off Australian forces under the Five Eyes arrangement - which also includes New Zealand and Canada - about the plot that may have been carried out within days.
The ABC's 7:30 reports Australian police had hoped to gather more evidence before taking people into custody however the UK government said it would issue a public security threat if the raids did not happen immediately.
A UK Home Office spokesman said: "We do not comment on intelligence matters."
It's been reported communications intercepted from Syria were the source of the intelligence.
One of the men in custody has been named as Bulldog's supporter Khaled Khayat. He has not been charged but remains in police custody with three other men who are assisting inquiries.
The development raises questions for Prime Minister Malcolm Turnbull who refused to comment on whether the plot had been apparently missed by local agencies.
He said intelligence is a matter of "seamless co-operation" between allies and stitching together snippets of information to build up a bigger picture.
"We have very fine intelligence services. But, every nation, every nation depends on the work of others," he said on Monday.
"We are intensifying, all the time, our collaboration with other countries and other agencies. Because, you know, the threat, there are no borders; I mean, nowhere is far away from anywhere else nowadays.
"In an age of the internet and the age of social media and the age of over-the-internet messaging applications, Syria is not a long way away from Sydney."
The Daily Telegraph reports an Etihad flight to Abu Dhabi was the target of the alleged plot, which would have involved a gas substance rather than explosives as it is more difficult for authorities to detect.
It's feared the plot would have been carried out within days, however authorities are confident they have eliminated the immediate threat.
Over the weekend the New South Wales Joint Counter Terrorism Team (JCTT) conducted multiple searches at four addresses in Lakemba, Surry Hills, Wiley Park and Punchbowl.
Police have been combing the addresses for vital clues and could be seen combing through rubbish bins.
They have remained tight tipped about their discoveries however it's believed an improvised bomb may have been hidden inside a meat grinder and gas was involved. A note removed in a clear plastic bag contained the details of a flight from Sydney to Jakarta.
Border Protection Minister Peter Dutton said he did not want to go into detail but there was a "significant threat".
Prime Minister Malcolm Turnbull described the conspiracy as "a very serious terrorist plot" but refused to elaborate. He also wouldn't confirm reports an overseas tip-off triggered the raids.
"Good intelligence, great police work, great investigation, great co-ordination has enabled us to disrupt that plot," Mr Turnbull told reporters in Perth.
Australian National University criminologist Clarke Jones said the alleged details show the plot was "amateurish" in nature.
"If it was to be hand luggage, that's a very crude plan and shows a lack of experience and the fact that they might not have travelled a great extent," he told AAP.
Members of the community have been advised to continue as normal and report any suspicious activity to triple-0 (000), Crime Stoppers on 1800 333 000 or the National Security Hotline on 1800 123 400.