"We believe there are elements that are satisfied in terms of religious motivated extremism," NSW Police Commissioner Karen Webb said.

Another of the parish priests, Father Isaac Royel, was also injured in the alleged attack at Christ The Good Shepherd Church in Wakeley about 7.10pm on April 15.

A 16-year-old male was arrested and taken to hospital where he underwent surgery for hand injuries, and remains there under police guard.

Commissioner Webb said the alleged offender was known to police but not on any terror watch list, and police believe he was acting alone.

She said the bishop and priest who were allegedly stabbed were "lucky to be alive".

The Christian leader was delivering a sermon to a busy congregation in the evening on April 15 when he was allegedly attacked.

A 53-year-old sustained lacerations to his head while a 39-year-old man who attempted to intervene also suffered lacerations and a shoulder wound, police said.

Both were in a stable condition in hospital on April 16.

After the alleged attack a large crowd which grew into hundreds "turned on police" who were called to respond, Commissioner Webb said.

"Police officers were doing their job and they were attacked," she said.

"People used what was available to them in the area including bricks, concrete, palings, to assault police and throw missiles at police."

A constable sustained a broken jaw after being hit with a brick and a fence paling while another was hit with a metal object, sustaining a twisted knee and a chipped tooth, police said.

Twenty police cars were damaged - 10 which were "unusable".

"This is unacceptable and those that were involved in that riot can expect a knock at the door," Commissioner Webb said.

Paramedics treated 30 different patients with seven taken to hospital, but when the crowd turned they were forced to shelter inside the church for three-and-a-half hours, NSW Ambulance Commissioner Dominic Morgan said.

"Six of our paramedics could not leave that facility for fear of their own safety from the community that they serve," he said.

"This is outrageous."

In a statement parish priest Father Daniel Kochou said the church pleaded with its followers to "keep peace with one other and your neighbour", and denounced any retaliation.

NSW Premier Chris Minns also called on the Sydney community to "remain calm" and let police do their job.

"There is no such thing in Australia as taking the law into your own hands," he said.

"You will be met by the full force of the law if there is any attempt for tit-for-tat violence in Sydney over the coming days."

Mr Minns convened a meeting of faith leaders representing religious communities across western Sydney at 10.30pm on the night of the alleged attack.

"All of the following community leaders endorsed and supported a unanimous condemnation of violence in any form, called for the community to follow first responder and police instructions and called for calm in the community," he said on X.

Local MP Dai Le said the declaration of a terror attack meant people in the local community would not feel safe.

"As political leaders of our community to go so quickly to declare that in a community like ours, I am very concerned for the safety - it's actually doing the opposite, you're not going to be able to create cohesion with those language and those words," she told ABC News Breakfast.

"I hope they better have a very good reason for that."

The Australian National Imams Council "unequivocally condemned" the attack on Bishop Mar Mari Emmanuel and urged the community to stay calm.

"These attacks are horrifying and have no place in Australia, particularly at places of worship and toward religious leaders," the council said in a statement.

A joint counter-terrorism investigation has been established including NSW Police, the Australian Federal Police and ASIO (Australian Security Intelligence Organisation).

Prime Minister Anthony Albanese said it was a "disturbing incident" and there was no place for violent extremism in the community.

"We are a peace-loving nation. This is a time to unite not to divide as a community and as a country," he said.

"I understand that people are feeling uneasy and that is understandable given the atrocity that occurred [at Bondi Junction on April 13] and then this incident last night."

ASIO director general Mike Burgess said the attack appeared to be religiously motivated but the investigation was continuing.

"Our job is also to look at individuals connected with the attacker to assure ourselves that there is no one else in the community with similar intent," he said.

He said the terrorism threat level remained at "possible" and it would not change as the result of one incident.

AFP Commissioner Reece Kershaw said the attack would have a "significant impact on the Australian community".

"The community can have confidence in the joint counter-terrorism team and how successful they have been over the years in tracking down and prosecuting these offenders," he said.

But he urged all Australians to be vigilant and report anything they see or hear that was "not right" to the national security hotline 1800 123 400.

Opposition leader Peter Dutton wished the bishop a speedy recovery and acknowledged the parishioners who went to his aid.

"I want to make it very clear that an act of violence in a place of worship is completely unacceptable in our country," he said.

"It doesn't matter whether you disagree vehemently with a priest or an imam or with a rabbi, with any person, violence is never the answer."

Anyone with information about the Wakeley incident is urged to contact NSW Police or Crime Stoppers on 1800 333 000.