It is a ring of steel 18 months in the making, gathering pace in recent days with Black Hawk rotorblades thudding overhead as special forces commandos go through their paces, and traffic delays as police rehearse motorcade routes between the airport and the sprawling summit precinct downtown.

With nearly $100 million spent, about 6000 police on shift, 900 soldiers on duty and another 1000 on standby, authorities and experts say the chances of any ill-doer reaching one of the world's 20 most powerful leaders or their entourages are close to zero.

But there are still concerns. The danger lies beyond the ring of steel, where the "nightmare scenario" according to terrorism expert Greg Barton is a lone-wolf jihadist forming a spontaneous plot to attack a train station or a shopping centre somewhere else in Brisbane and leveraging the publicity of G20 for maximum global impact.

"You don't have to go into downtown Brisbane to the G20 precinct. An attack can happen a long way from the meeting itself and it will still get reported around the world as being an attack at the G20," he said.

ASIO and the Australian Federal Police have a good handle on jihadist networks and it is understood none of the known groups are directing attention to the G20. But one of the major concerns among counter-terrorism agencies in the Islamic State-era is the trend towards spontaneous, low-tech attacks.

A recent directive from the militant group in its magazine Dabiq told would-be terrorists not to talk too much about their plots, nor even think about them, but rather go ahead and do them.

That, according to Professor Barton, makes intelligence-gathering hard.

"The trouble with intelligence is that you really do depend upon people you know about talking to other people or be able to track some sort of chatter.

"My perfect storm concern would be someone saying, okay G20 is big, I'm going to give my life to this, I'm not going to ask for advice, I'm not going to plan, because they'll be listening, I'm not even going to tell my fellow extremists. I'm just going to go and do something. They'll know what I've done when I've done it … I'm going to try my luck at this train station or this mall. I'm sure that's what the police and intelligence agencies are most worried about."

Law enforcement sources have also expressed concern about that scenario.

Then there's the fact that even rings of steel have flaws, as shown by the Chaser prank at the 2007 APEC Summit, when the comic troupe surprised even themselves by penetrating well beyond the security perimeter.

Security intelligence expert Roger Henning, the chief executive of Homeland Security Asia Pacific, said a lone wolf attacker not concerned about his own life could still cause mayhem.

"A show of force will not deter a committed suicide bomber because they are unafraid and seek martyrdom," he said.

But one security expert with extensive experience keeping major international events safe said gaffes such as the Chaser episode helped police and security agencies improve. He said Australian authorities – including Queensland Police who protected the 2002 Commonwealth Heads of Government Meeting at Coolum shortly after the September 11 attacks – have a "tremendous track record … of being able to protect major international summits", he said.

The strength at G20 would be "constant lessons learnt from the (previous events) and preparation, testing and rehearsal exercises beforehand", he said.

Beyond terrorism, Queensland police have received notice of 26 planned protests scheduled to take place during the G20 summit, however they have declined to name the groups.

Of the protest groups publicising their demonstrations on social media, none have overtly suggested they want to clash with police.

However it could be a loosely connected network of like-minded protesters who cause the most trouble, says Janet Ransley, the head of Griffith University's School of Criminology and Criminal Justice.

Black Bloc demonstrators generally do not belong to specific groups, and the term Black Bloc is commonly used to describe their protest style of wearing all-black, trashing buildings and clashing with police.

Associate Professor Ransley said it was "naive" to think Black Bloc protesters – who have attended other similar events – would not be in Brisbane.

"The challenge for police is to work out which ones are the real danger and which ones simply want to be there," she told Fairfax Media this week.

The only dissent appears to be from Plan B. According to its website hosted on a popular blogging domain, Plan B's motto is "disrupt, resist and obstruct" the G20.

Plan B doesn't list any contact details online and Fairfax Media has been unable to find any evidence that the group has any members.

"Instead of gathering our strength and marching into the traps they have set for us, we are calling for disseminated disorder. Rather than an arterial block, we want to see a distributed attack on the peripheries," Plan B's website states.

Brisbane Anarchist group Autonomous Action is expected to attend, but distanced itself from a poster it uploaded online which called on people to "smash the G20".