Adani's proposed coal mine in Queensland would not worsen Australia's greenhouse gas emissions, according to federal Labor leader Bill Shorten.
When asked when there would be enough damage from climate change - such as harm to the Great Barrier Reef - to prompt Labor to block the mine, Mr Shorten said he didn't accept the proposition.
"I believe that our policies on renewable energy will actually reduce our emissions. The actual decision about Adani is not going to affect Australian emissions," he said after a speech on Thursday.
"The IPCC (Intergovernmental Panel on Climate Change) has made it clear that the way you take global action isn't through particular projects.
"So we are very committed to protecting the reef, we are very committed to taking action on climate change, and we've made it very clear that we won't put any taxpayer resources into the Adani mine."
His comments came as dozens of activists opposed to the Carmichael coal mine in the Galilee Basin rallied outside the Sydney event.
Others formed queues on Thursday morning outside Mr Shorten's Melbourne office and the offices of senior Labor MP Anthony Albanese in Sydney and Queensland Premier Anastacia Palaczszuk.
Brisbane-based Stop Adani activist Anne Gardiner said Labor's energy policy lacks credibility if it allows the Adani mine to go ahead.
"With the impacts of climate accelerating, the world just can't afford to mine and burn more coal or allow unprecedented volumes of water to be extracted from inland rivers and precious water basins," she said in a statement.
Mr Shorten has previously said there is still plenty of scepticism that the controversial project will ever proceed.
Adani, an Indian mining giant, revealed earlier this month it had scaled back its plans for the mine as it tries to stitch up funding for the project.
The original $16.5 billion Carmichael coal mine planned to export 60 million tonnes of thermal coal each year, but capital costs have now been scaled back to less than $2 billion for the first stage.
Australian Associated Press