"Once we have a situation when we have Australians being recruited and travelling to the Middle East to fight for ISIS, inevitably something is going to happen here and unfortunately it looks like it has," he said.
A black flag, which appears to be the Shahada flag adopted by ISIS, has been pressed against a window of the cafe where a a single gunman has held people hostage.
Australian National University visiting Professor Clive Williams said police would try to negotiate with the gunman but, depending on his intentions and weapons, it was hard to say how it would end.
Prof Williams said the person was clearly after publicity, which was why the central location was chosen. "It is inevitable. You are not going to be able to say to the media don't cover this because it is counter-productive," he said.
Prof Williams said police would seek to establish an outer cordon to keep people well away and also limit media coverage of their preparations.
Hostages pressed against the window in the Sydney Lindt cafe. Photo / Channel 7
Prof O'Brien said world media would be watching and this could go on for days.
"Police will be there assessing how they can deal with the problem. The first step will be negotiation and then something harder and they'll even be thinking of handing it over to the military," he said.
Scott Stewart, security analyst for US intelligence group Stratfor, said he was surprised there hadn't been more such attacks given repeated calls by jihadist leaders and their ease of execution.
ISIS spokesman Abu Muhammad al-Adnani made a plea to followers in September, calling for attacks in nations allied against ISIS - US, UK, France, Germany and Australia.
Watch: Raw: Sydney CBD shutdown by siege
"If you are not able to find an IED or a bullet, then single out the disbelieving American, Frenchman, or any of their allies. Smash his head with a rock, or slaughter him with a knife, or run him over with your car, or throw him down from a high place, or choke him, or poison him," he said.
Mr Stewart said lone assailants could easily fly under the law enforcement radar, especially if they avoided alerting authorities through internet activity and social media postings.
"In most cases, these kinds of individuals can be highly successful in carrying out an attack, especially a simple attack directed against soft targets," he said in a recent analysis.
- For more go to news.com.au
