Bill Ryan, a veteran of the Kokoda campaign against the Japanese, says Australia's future depends on winning the battle against the giant coal mines proposed by Adani and other miners in Queensland's Galilee Basin.

"We went away to save Australia," Mr Ryan, now 95, said outside the Sydney offices of mining contractor Downer on Tuesday. "If we don't put a stop to these megamines…we'll end up with a great catastrophe for Australia."

Mr Ryan was one of about 15 protesters from the Galilee Blockade group who tried to meet Downer chief executive Grant Fenn and other executives to encourage the firm to drop its "preferred contractor" status with Adani.

The Indian-owned miner wants to open up the Galilee with its proposed $16 billion-plus 60 million-tonne-a-year Carmichael mine.