"The fact that they might be causing harm to individuals is in some respects - I know this is a horrible thing to say - it's incidental to that major project of just being noticed, of just having their cause announced through the media. "The longer the group has publicity, the closer they are to achieving their objectives. They'll want it to be as protracted as possible, they'll want to cause as much threat as possibly they can...and they'll run this until the publicity fades." Hostages are being forced to hold an Islamic flag against the window of the Lindt Chocolat Cafe in Martin Place, which was stormed by at least one gunman on Monday morning. About 10 staff and customers are being held at gunpoint inside the cafe, initial reports suggest. It is understood police have asked Channel 7 to turn their cameras away from the front of the Martin Place cafe. Professor Lewis said the siege may have been plotted and carried out by as few as two or three people and possibly triggered by Monday morning's terror raids in Sydney.

"It's certainly an interesting coincidence that there was terrorism raid this morning and now this action is taking place," he said. "What's happened in the last couple of months is that Islamic State has sent its messages around to supporters and potential supporters around the world and asked them to act - to do things that they have been planning or thinking about doing... "It is a time when Islamic State is encouraging these kinds of more random approaches to action." It is not yet clear whether the gunman, or gunmen, are associated in any way with Islamic State or any other proscribed extremist group. Professor Lewis said extremists also used acts of terror to promote their cause and demonstrate their power to potential supporters.

"They want to threaten the people who they regard as their enemies - that is the people of Australia and the government of Australia - but also to promote and attract supporters...or potential supporters," he said. In September, the government raised Australia's terror threat from medium to high, meaning an attack on home soil was considered "likely". Prime Minister Tony Abbott has said that security agencies had not detected any particular plots, but the raised alert level meant there were people in Australia with the "intent and capability" to carry out a terror attack. Then-ASIO boss David Irvine said the threat to Australia had increased "because of the numbers of people we are now having to be concerned about in Australia, because of the influence of Syria and Iraq on young Australians, both in terms of going to those places to fight, but also in terms of what they are doing here in Australia with a potential intent to attack". Professor Lewis said security forces in Sydney would likely be trying to assess the extremists' commitment to martyrdom.

"We don't know who these people are...we don't exactly know what their motivations are and that I guess will unfold during the day and we'll have a better view of what their commitment to martyrdom might be," he said. "These actions are very, very serious actions and I expect that if they're taking that kind of radical step, their idea of martyrdom is probably at the front of their minds." The extremists chose their target carefully, he said. "It appears they've chosen a target - what we'd call a soft target, which is fairly easy to appropriate - and they've done so in one of the most visible parts of Australia [in Sydney's CBD]," he said. "It's a significant government zone, but it's also opposite the Channel 7 news studios. That's the ideal [place] for a news-loving extremist group."