Three judges will sleep on whether a pro-Palestine rally can go ahead safely at an iconic Australian landmark, amid fears more than 40,000 will be squeezed into a "narrow funnel".

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 historic

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