Authorities say they are prepared for every contingency, including violent protests on the streets of Brisbane, as they prepare to lock down the G20 host city as part of an unprecedented security operation.
Military-style checkpoints will be set up and 6000 police deployed around Brisbane to help secure the summit and the world's most powerful leaders, while access to large parts of the Queensland capital will be restricted.
The massive $100 million security operation will include 900 soldiers stationed around the city, some using under-car camera, mobile X-rays and explosive detection equipment, and another 1000 on stand-by.
Police have been given extra search powers for the duration of the event.
The summit, which involves the leaders of world's 20 biggest economies, comes in the wake of the terror alert being raised amid fears of an attack because of the number of Australians "fighting with and supporting" terrorist groups in the Middle East.
Authorities are concerned too about the risk of violence from protest groups, and have been working with known groups for 18 months in a bid to avoid a repeat of past G20 when streets erupted into riot.
In 2010, at the G20 summit in Toronto, Canada, demonstrators associated with the Black Bloc movement set police cars alight and smashed bank windows.
A spokesperson for the Queensland Police said authorities had been working for 18 months to ensure the city was secure.
"The service will facilitate peaceful and lawful protests in the lead-up to and during the G20 Leaders' Summit, and we have been liaising with groups to ensure they can achieve their desired protest outcome in a safe way that does not disrupt the security arrangements ...," the spokesperson told AAP.
"The Queensland Police Service is not anticipating any violent protests to occur during the G20 period, however we are prepared for every contingency, should they arise."
Professor Clive Williams, a former director of security intelligence with the Australian Defence Force and an expert on terrorism, says it's "issue motivated groups" that are the concern.
"I think the terrorism threat is probably less likely," Prof Williams said.
"It's pretty unusual because security is usually high and it doesn't tend to attract those types of people. There are lots of easier targets around the place."
Prof Williams said "fixated individuals" are also a category of concern, pointing to when student David Kang in 1994 rushed a stage at Darling Harbour in Sydney and fired two blanks from a starter's pistol at Prince Charles.
"They're lone-wolf actors. But they can also be a nuisance when they have an issue they want to promote," Prof Williams said.
Authorities have also downplayed the possibility of financial institutions being targeted, as has been the case at previous G20s.
Deputy Police Commissioner Ross Barnett has said there was no credible threat of attacks on financial institutions or their staff.
"There had been general postings on social media by unidentified people targeting the theme of capitalism and greed in G20 nations," he said.
