Charles Sturt University (CSU) counter-terrorism expert, Associate Professor Nick O'Brien is not surprised a siege in Martin Place appears to have links to the Islamic State movement.

Listen to Professor O'Brien's comments here.

Professor O'Brien is Head of CSU's Australian Graduate School of Policing and Security (AGSPS) and has a background in international and counter terrorism.

"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," said Professor O'Brien.

"The world media will now be watching Sydney and what Australia does. It will be extremely worrying for everyday Australians and is likely to prompt a discussion about raising the security threat level.

"This will paralyse the Sydney central business district, it could go on for days, the 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."