Police in Australia say a knife-wielding man who allegedly attempted to stab several people in Sydney was "lone actor" who may have had some terrorist ideologies.

A 21-year-old man who was pinned down and held by members of the public has been arrested and is being questioned.

The body of a 21-year-old woman was found in an apartment building near the scene, at a busy intersection in Sydney.

Another woman aged 41-years-old was stabbed in the back in the street and has been taken to hospital where she is in a stable condition.

Terrorist ideologies

It is believed the man was carrying a USB drive containing documents promoting terrorist viewpoints with references to recent attacks in the US and New Zealand.

However, investigators said he has no known linked to terrorist groups and is believed to have been a lone actor.

Police said he used the phrase "Allahu akbar" - Arabic for "God is great" and witnesses said he was shouting about religion before calling on police to shoot him.

NSW police commissioner Mick Fuller said: "Information was found on him that would suggest he had some ideologies related to terrorism... but he has no apparent links to other terrorist organisations."

'No remorse'

The suspect was pinned down and restrained by three men from Manchester in the UK - Lee Cuthbert, Paul O'Shaughnessy and his brother Luke.

Paul O'Shaughnessy, a former footballer for Bury FC, said: "We saw what was going on and chased the man.

"He had a knife, it was covered in blood. He kept saying something in Arabic, he was clearly under the influence of something.

"My brother Luke managed to keep him pinned down until the police arrived. My brother put a crate and a chair to keep him pinned down. He didn't show any remorse at all."

"Heroes"

NSW Police Superintendent Gavin Wood described the three men as heroes.

"They are significantly brave people," he said. "They approached the person... with clear evidence of a stabbing previously.

"These people are heroes, and I want to acknowledge that."

The motivation for this attack has not yet been determined as Police are continuing with their enquiries. Any further offical information will be provided by the New South Wales Police, who are keeping us appraised through our agencies, including details of casualties.

— Scott Morrison (@ScottMorrisonMP) August 13, 2019

Australian Prime Minister Scott Morrison said: "The violent attack that took place in Sydney this afternoon is deeply concerning.

"The attacker is now in police custody following the brave actions of those who were present at the scene and were able to able to restrain him."

Additional reporting from IRN