The NSW Court of Appeal on Wednesday heard a last-minute police challenge to the Palestine Action Group demonstration on Sunday, slated to start in Sydney's city centre and finish under the sails of the Sydney Opera House.
Organisers initially suggested 10,000 people would attend before telling the hearing they now expected 40,000.
Chief Justice Andrew Bell scrutinised the estimated figures, comparing them to the massive Sydney Harbour Bridge protests in August where a crowd between 90,000 and 300,000 marched through rain.
"This site (Opera House) is more iconic, the forecast is good, the occasion is historically more potent ... the likelihood of significantly more than 40,000 (is) a realistic possibility," he said.
He went on to illustrate possible crowd crush scenarios, referring to the Hillsborough stadium tragedy in the UK in 1989.
Nearly 100 died during a stampede minutes before an FA Cup semi-final between Liverpool and Nottingham Forest.
Justice Bell said Macquarie Street - a major city route - could turn into a "narrow funnel" where demonstrators are bunched into a tight space.
That chimed with Assistant Police Commissioner Peter McKenna's testimony that the march "has disaster written all over it".
But the organisers' barrister, Felicity Graham, told the court previous unticketed events like Vivid were monitored by the Opera House and had been managed capably.
"That can be done on Sunday," she said.
"That information can be relayed effectively live to the police (and it) can be managed in the same way that a light show is managed."
She noted the urgency and timing of the protest was vital in a democracy.
NSW has a permit system that allows protest participants to block public roads and infrastructure unless a court denies permission after a police challenge.
Palestine Action Group has been organising weekly rallies for two years since Israel's military assault on Gaza began in 2023.
Police say the Opera House forecourt, which is mostly surrounded by water, is ill-equipped to handle a large, swelling assembly with loudspeakers blasting chants and mass groups of people filtering at different times.
"I don't think people are going to come walk through and then just walk out again. That's just farcical," Mr McKenna said, cautioning outside groups would try to disrupt it for their own purposes.
In a rare move, a peak Jewish body was permitted to intervene in the case, recounting how an unauthorised protest under the Opera House  sails on October 9, 2023 still reverberated widely.
Some participants chanted anti-Semitic slurs and an Israeli flag was burned, in footage broadcast globally.
"The proposed public assembly will cause further fear of anti-Semitism, which includes violence and further distress amongst the Jewish community," Vanessa Whittaker, barrister for the Executive Council of Australian Jewry, told the court.
The court is due to rule on police's prohibition application on Thursday morning.
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