A 16-year-old boy has been arrested after Bishop Mar Mari Emmanuel was stabbed at Christ the Good Shepherd Church in Wakeley on Monday night, with the assault captured during a live-streamed sermon.
Director-General of Security Mike Burgess said the spy agency believed the teen acted on his own but an investigation would look at all avenues.
"It does appear to be religiously motivated but we continue our lines of investigation," he told reporters in Canberra on Tuesday.
"At the same time, our job is to look at individuals connected with the attacker to assure ourselves there's no-one else in the community with similar intent."
Mr Burgess said Australia's national terrorism threat level would remain at "possible" as one incident wouldn't cause it to change.
The national security committee has met following the stabbing with a joint counter-terrorism task force set up with federal police and ASIO.
Mr Burgess said while tensions around the war in the Middle East resonated in Australia, ASIO had not seen evidence of individuals motivated by the conflict to carry out acts of terror.
Broken glass litters lawn in front of the Christ The Good Shepherd Church in Sydney. (Bianca De Marchi/AAP PHOTOS)
He urged Australians to be mindful of the language they used with rising tensions in the community.
Terrorism Research Centre director Clive Williams said an alienated young person could easily become indoctrinated sitting in their bedroom reading the internet.
"It's very easy for them to do that without anything being indicated to anybody, even the parents may not realise that the person is thinking along those lines," he told AAP.
"It's unlikely there'll be others involved, because if there were others involved, it would usually be more organised."
Professor Williams said young people being radicalised happened "unfortunately too often".
Anthony Albanese has condemned the violence and attacks on police and police cars. (Paul Braven/AAP PHOTOS)
Prime Minister Anthony Albanese said there was no place for extremism in Australia.
"This is a disturbing incident, there is no place for violence in our community, there's no place for violent extremism," he said.
"We're a peace-loving nation, this is a time to unite, not divide, as a community and as a country."
Mr Albanese said the government remained concerned about the role of social media with the spread of graphic videos and images appearing online from the attack.
"I'll just say to people 'think before you press send' because this can have a really disruptive impact on people," he said.
Opposition Leader Peter Dutton said acts of violence at places of worship were completely unacceptable.
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