Updated 3.05pm

Four people are being treated for "non-life threatening injuries" after a stabbing at a church service in Sydney on Monday, emergency services said.

Australian police said they had arrested one man.

The incident came two days after a man with a knife killed six at a shopping mall in the east of the city.

A video stream showed Monday's gruesome attack at an Assyrian church unfold in real time.

The video showed a man dressed in black approaching a bishop at the dais, raising his right arm and slashing at the preacher with a knife, prompting panic and screams among the congregation. 

Several people appeared to rush to help subdue the attacker.

The ambulance service told AFP that four men aged between 20 and 70 were being treated for injuries, including lacerations. 

"The injured individuals suffered non-life threatening injuries and were treated by New South Wales Ambulance paramedics before being conveyed to hospital," police said.

"A male was arrested and remains in police custody."

AFP verified the video as being taken at the Christ the Good Shepherd Church in Sydney's western suburb of Wakeley.

The neighbourhood is a hub for Sydney's small Christian Assyrian community, many of whom fled persecution and war in Iraq and Syria.

The church holds a prayer session every Monday evening.

Police said they began to receive emergency calls from the scene "about 7.10 pm".

They urged the public to avoid the area.

Australians are still reeling from Saturday's stabbing, which was carried out by a 40-year-old man with a history of mental illness.

In that attack, videos shared on social media showed unshaven itinerant Joel Cauchi pursuing mostly female victims as he rampaged through the vast, crowded Westfield shopping complex in Bondi Junction on Saturday afternoon.

A black ribbon was projected onto the Sydney Opera House on Monday as a mark of respect for the victims.