The trio, named in local media as Richard Green, 74, his wife Carolyn, 71, and documentary filmmaker John Davis, 72, left the small village of Breeza 390 kilometres (242 miles) north of Sydney for the Greens' home on the city's northern beaches on Saturday.
A search began after the Greens failed to return home and the wreckage was discovered late Monday in Watagans National Park in an "inaccessible" and "fairly rugged location", local New South Wales police commander Superintendent Craig Jackson said.
The festival at Breeza was a protest against the Australian government's approval of the huge Chinese-run Shenhua coal mine, which is located near prime farmland in the state.
The approval, which was also criticised by national Agriculture Minister Barnaby Joyce, came amid growing community concerns about large-scale mining in rural areas and valuable agricultural and mineral assets being owned by foreigners.
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New South Wales Greens MP Jeremy Buckingham, who spent time with the trio at the festival, described them as "committed environmentalists who did everything they could to record and protect Australia's wonderful environment".
The Australian Transport Safety Bureau will investigate the crash.