In a decision delivered on Thursday, the court ruled in favour of police, who moved to have the protest planned for the Port of Newcastle from November 22 to 24 deemed an unauthorised assembly.
Such an order leaves protesters liable to arrest if they blocked port traffic or otherwise obstructed the facility.
In his ruling, Justice Desmond Fagan described the impact of the planned action as "excessive".
"A 30-hour interruption to the operations of a busy port is an imposition on the lawful activities of others that goes far beyond what the people affected should be expected to tolerate in order to facilitate public expression of protest and opinion on the important issues with which the organisers are concerned," he said.
Former federal environment minister Peter Garrett is scheduled to perform at the "protestival". (Jason O'BRIEN/AAP PHOTOS)
Hundreds of protesters were expected to take to the water, while more than 5000 people could attend the event on land.
The "protestival" would feature live music, including a scheduled performance from former Midnight Oil frontman and federal environment minister Peter Garrett.
Organisers Rising Tide said the event would be one of the most significant climate uprisings on the planet.
During a final hearing on Wednesday, Justice Fagan raised concerns about the safety of the event and the ability of police to maintain order during a 30-hour protest on water.
He questioned whether it was wise to leave police without powers to break up the gathering in circumstances of public safety, describing it as "unpredictable".
The judge raised concerns about a 30-hour protest on the water. (Jeremie Canavesio/AAP PHOTOS)
If the police request was refused, protest participants would be immune from arrest and prosecution, Justice Fagan added.
Neal Funnell, representing Rising Tide, noted organisers had banned drugs and alcohol from the event, adding police would still maintain arrest powers in response to violence.
"If the concern is that there's a possibility that a violent disorder may occur ... the police wouldn't be constrained in their powers," he said.
But NSW Police on Thursday said the court decision validated concerns about the safety of both the water and land-based components of the planned protest.
"For their own safety and that of the other users of the port, we request that people who still attend this event refrain from entering the harbour on kayaks or other vessels with the intention to obstruct other users of the port, or engage in other forms of civil disobedience," a statement said.
Organisers previously submitted an application for a shorter 30-hour blockade, rather than the original 50 hours, while also halving the time protesters would be camped in the area to four days.
Following a similar 30-hour blockade in 2023, more than 100 people were arrested after refusing to exit the water when the police-approved protest ended.
The Port of Newcastle is the world's largest coal export port, sending about 144 million tonnes of the fossil fuel worth $38 billion offshore each year.
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