Veteran conservationist Bob Brown has compared Adani's Carmichael coal mine to Tasmania's quashed Franklin Dam, slamming the "destructive wealth and arrogance" of the company's chairman.
The former Greens leader joined protesters from the Stop Adani group in Sydney on Saturday where he demanded no public money be spent on the Queensland project.
Mining tycoon Gautam Adani this week declared the company would break ground on its controversial $16.5 billion coal mine in Queensland in October.
"This is the biggest environmental, heritage, Indigenous and lifestyle issue I have seen come along in decades in Australia," Mr Brown told reporters at the summit.
He said Mr Adani had signalled, in a "heightened arrogance", that a billion-dollar loan for the project from the Northern Australia Infrastructure Fund was already locked in despite no public announcement from the Turnbull government.
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"You're not welcome to bring your destructive wealth and arrogance to ride over the majority opinion of Australian people who don't want you to have that loan and won't let you get away with that mine," Mr Brown said.
He predicted a revolt at the next election if the loan and "evil, rotten, corrupt" mine went ahead.
Mr Brown rose to prominence as director of the Tasmanian Wilderness Society as it campaigned against the Franklin Dam in the late 1970s and 1980s.
It was a battle won by conservationists and Mr Brown warned Carmichael mine opponents were similarly prepared to physically sit in front of machinery.
Maggie McKeown from the Mackay Conservation Group said Queenslanders had seen the impacts of climate change in the form of heat, coral bleaching on the Great Barrier Reef and cyclone damage.
"If Adani opens up the coal in the Galilee Basin, it's undeniable that these events will become more frequent and more intense," she said.
Mine opponents argue the project cannot proceed because carbon emissions from the coal being burned in India will further damage the already-ailing reef through climate change.
The Federal Court last week dismissed two legal bids to stop it going ahead, from traditional owners and environmental groups.
Queensland Premier Annastacia Palaszczuk has ruled out financial support but her Labor government views the enormous project as a valuable jobs generator.
Federal Opposition Leader Bill Shorten has been accused by protesters of sitting on the fence on the issue.
The Stop Adani group will hold a national day of action against the project on October 7.
Prime Minister Malcolm Turnbull's office declined to comment.
