The lawyer of a Sydney man held by police for four days on suspicion of involvement in an alleged terror plot to down a plane says he will "review police action" after his client was found to have no involvement.
Abdul Merhi, 50, was released by police at about 7pm on Tuesday night without being charged, police confirmed on Wednesday morning.
Three other men, including his brother Khaled Merhi, remain in custody over the plot after police were granted permission by a court to hold them for up to a week without charge.
The men are being held on suspicion of hatching a plan to down a plane using a bomb or noxious gas inside a meat mincer with assistance from Islamic State in Syria.
In a tweet, Merhi's lawyer Moustafa Kheir described his client's time behind bars as a "tough few days".
"But he's relieved the truth is out."
Fairfax Media with AAP has reported that Merhi heard his parents' street was being shut down on Saturday and went to have a look himself, and was then arrested. His lawyer told AAP that he felt like he was "in a movie" when he heard about the allegations of a plane plot.
"It's a very serious allegation to have against you. There's a lot of stress associated there, and not knowing, and he was shocked that he was being questioned.
"It's just unfathomable that he would be associated with anything like this."
The lawyer said he would be investigating "what basis (the police) had had to do what they did".
But Prime Minister Malcolm Turnbull said it was important to respect the investigation process, and urged the public not to "draw any conclusions" beyond the man's release.
"There will be a lot more said if, and when, charges are laid... but at this point it is very important to respect the integrity of the investigation process," he told reporters from Albany in WA.
"My focus, my relentless focus, is on keeping all Australians safe."
It's understood that the IS-linked plot was uncovered through tip offs from intelligence agencies in the UK and the United States, prompting police to bring forward raids on properties in four Sydney suburbs. But the Turnbull government has vehemently denied that it was pushed into the raids by British threats to issue a public travel warning about Australian flights.
The four men who were initially arrested -- Khaled Khayat, Mahmoud Kayat, Abdul Merhi and Khaled Merhi -- reportedly have deep connections to Islamic State, including relatives on the front lines in Syria.
Neil Fergus, Chief Executive of the Intelligent Risks Group, has labelled the planned raids the "most significant aviation security threat that we have had in this country for many years."
It has led to ramped-up security at all of Australia's major airports, leading to long queues and extended wait times as passengers were warned by airlines to arrive early for flights.