An Australian man accused of attempted murder after stabbing another man outside Sydney was "inspired by ISIS", authorities said Sunday. 

Ihsas Khan, 22, was ordered held without bail after he allegedly attacked the 59-year-old victim while he was walking through a park in one of Sydney's western suburbs Saturday. 

The unidentified victim was airlifted to a local hospital and listed in critical condition. 

Police said Khan also attempted to stab an officer before his arrest. He faces a life sentence if convicted. 

New South Wales Deputy Police Commissioner Catherine Burn told reporters Sunday that initial investigations showed there was 'clearly some planning' involved prior to the attack.

"This was clearly a very volatile, a very violent situation that police and the members of the community were confronted with," Burn told reporters.

Khan shouted words at the scene, which led police to believe he was inspired by ISIS, Burn said.

"We know that this person has strong religious beliefs inspired by ISIS. What made him act yesterday we don't know," she said.

Khan had previously been charged with a property-related offence, she said.

"He has had a couple of interactions with local police over matters where we might say his behavior was odd or unusual," Burn said. "He is a person of concern."

Despite his unusual behavior, he was not known to be connected with any terrorist group, she said.

"This really highlights the challenge that this is the new face of terrorism," she added.

Federal Attorney General George Brandis said bystanders had put their own lives at risk by intervening in the attack.

"It may very well be but for the bravery of those citizens to intervene, the victim's life would have been lost," Brandis told reporters. "They are an inspiration to us all. They acted heroically and we should all be in their debt."

An 18-year-old man was arrested on Thursday outside the Sydney Opera House after allegedly telling security guards he was under instructions to carry out an attack by ISIS.

Counterterrorism investigators charged the teen with threatening to destroy property. Police said he was was carrying two canisters of automotive fluid.

His arrest came days after ISIS urged followers to stab, shoot, poison and run over Australians at iconic locations including the Opera House.

The teen appeared in a Sydney court on Friday, where the judge ordered him to undergo a psychiatric assessment.

The Associated Press contributed to this report.

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