Why police and protesters disagree on how big Sunday's Gaza rallies were

Tens of thousands of Australians joined pro-Palestinian rallies across the country on Sunday, but the exact number of protesters at the events remains unclear.

The protests have grown in both size and frequency in recent weeks as relations between Israel and Australia fractured following the government's decision to recognise a Palestinian state.

Organisers and police disagreed on the total in attendance at the protests in Brisbane, and mixed numbers were shared for other events in other capital cities.

At the event in Brisbane, organisers Justice For Palestine Magan-djin said around 50,000 people were in attendance, but Queensland Police disputed that, saying only 10,000 were there.

The police would have had a reason to say the numbers they did, according to Professor Alison Hutton, a lecturer at Western Sydney University and an expert in mass gatherings in a crowd.

"You would consider that the police have more experience in this space," she said.

Disputed numbers

More than 40 protests took place across Australia on Sunday. Justice For Palestine Magan-djin and Palestine Action Group said approximately 350,000 attended the rallies, including around 50,000 in Brisbane.

Police estimated the numbers there were closer to 10,000.

Organisers estimated the crowds in both Melbourne and Sydney reached 100,000 people.

However, police in both cities said the local number was around 10,000.

An approximate 20,000 turned out in Perth, 15,000 in Adelaide, and there were smaller protests also in Canberra, Hobart and other cities, organisers said.

Queensland's Acting Assistant Commissioner Rhys Widlman said police were confident of their estimate for the Brisbane protests.

He said Queen's Gardens — where the march began — can hold a maximum of 7,000 people, "for a fact".

"The fact that the park was full and the fact that some of the crowd spilled out into the surrounding roadways … is a very good indication for us that we know it's 10,000," he added.

How do you calculate crowd numbers?

"It's pretty easy to determine the scale of a crowd," Professor Hutton said.

"What you do is you measure the density per square metre," she said, adding that officials would take images or known sizes of event spaces into account.

She said counts were based on visuals, so an image of the crowd showing how many people were there and how close together they were would determine the number of people, on average, per square metre.

Dr Aldo Raineri, a senior lecturer at CQUniversity and an expert in crowd safety management, said that counting crowds required some planning and prior knowledge.

"Effective crowd management requires preparation and logistical consideration," he said.

He added that police would have prior knowledge of certain locations and an understanding of how big a crowd could fit in those locations.

Potential for a crowd crush?

During the huge protests in Sydney in early August, NSW Police released a statement asking protesters to "stop proceeding across the bridge to avoid a crowd crush at the northern end of the harbour bridge", before asking them to turn around and walk back toward Wynyard.

In Brisbane on the weekend, the protesters agreed to take a different route due to police concerns of a safety risk if they crossed the Story Bridge.

A Queensland court ruled that police had demonstrated there would be safety risks and crowd crush potential should the march over the bridge go ahead, due to the size of the group.

A crowd crush is defined as a "group of people in a square metre, or an environment where there are too many people for that space, that then restricts people's movements", explained Professor Hutton.

"For sure, you can fear a crowd crush in situations like this," she told the ABC.

Dr Raineri said the number of people in a space to determine a crowd crush was any more than 4 people per square metre.

"Such a situation involves people not being able to move at their own pace, in being swept along with the momentum of the crowd," he said.

"At densities greater than that, it also becomes difficult to breathe as people are literally crushed against each other or against a hard barrier."

The organisers of the protests acknowledged the scale had become bigger than expected but said all the protests had been peaceful.

Professor Hutton said from the protests she saw over the weekend, they were "cohesive crowds", which she defined as "gathering for an agreed-upon and shared purpose".

"I believe it was a nice, safe crowd," she said, adding that people were "all there for the same thing".

Australia supports the right to protest

Bill Browne, Director of the Australia Institute's Democracy & Accountability Program, said NSW Premier Chris Minns's vocal opposition to the protest did not echo the majority of the Australian public.

Research from the Australia Institute finds that 79 per cent of Australians support the right to protest — and that 71 per cent believed that right should be protected by federal legislation.

"Anti-protest sentiment from the NSW government and NSW Police is out of step with the public, with Australia Institute research finding most Australians support new protest right protections."

The Palestine Action Group's organiser, Joshua Lees, said the demonstration was "beyond their expectations".

Nearly two years into a war that Palestinian authorities say has killed more than 60,000 people in Gaza, governments and humanitarian organisations say a shortage of food is leading to widespread starvation.

The war began when Hamas militants attacked southern Israel, killing about 1,200 people and taking 251 hostages.