Sydney siege gunman Man Haron Monis wanted to join a group - expert

Man Haron Monis. Photo / AP

Sydney's Lindt cafe siege may not have happened had Man Haron Monis been accepted by the bikie gang who twice rejected him, a crime expert says.

Australian National University terror expert, Clarke Jones, told an inquest into the Lindt Cafe siege on Wednesday that Monis, whose mental health problems made him unpredictable, wanted to belong to a group.

The siege gunman tried twice to join different chapters of the Rebels Motorcycle gang after buying a bike and changing his appearance, but was rejected and had his bike stolen.

"I wonder if (the Rebels) accepted his membership, would we be here today?" Dr Clarke said.

But he said Monis appeared to have found external justifications for his internal problems.

Professor Jones said it wasn't clear if Monis' original destination was the Lindt cafe or the nearby Channel Seven building "because of his issues with (the) Sunrise (program)".

Greg Barton, Deakin University professor of global Islamic politics, said he believed the siege was an act of terrorism.

ANU associate professor Rodger Shanahan said Monis, who had been diagnosed as a functioning schizophrenic, was facing mounting legal problems in the lead up to the siege.

"That's a great deal of stress for someone who has a violent past," he said.

Over the past few months the inquest has heard testimony from dozens of witnesses who have some knowledge of the Iranian refugee's chaotic 18 years in Australia.

On Tuesday, US terrorism expert Bruce Hoffman told the inquest the siege was "overwhelmingly" an act of terror and Monis was clearly a radicalised terrorist.

Counter-terrorism and radicalisation expert Kate Barrelle said Monis displayed many behaviours of a radical but also some that could be attributed to his poor mental health.

The inquest continues.

- AAP