The demonstrators, using a fleet of kayaks, were staging a floating blockade of shipping traffic at Australia's largest coal port, the Port of Newcastle, over the weekend.
The protest aimed to challenge the nation's dependence on fossil fuel exports, according to the organizer Rising Tide.
Authorities agreed to let the protest run for 30 hours, but made arrests after activists refused to leave the water once the deadline expired.
'Avoid catastrophic climate collapse'
"We chose to risk arrest because scientists are warning that to avoid catastrophic climate collapse we must urgently phase out fossil fuels," Rising Tide said in a statement. The group comprises Newcastle residents campaigning against human-caused climate change.
Among those arrested was 97-year-old Reverend Alan Stuart who said he was doing this for his "grandchildren and future generations" whom he did not want to leave a world with "increasingly severe and frequent climate disasters.”
The protest, which has taken place annually since 2016, called for an end to new coal projects and a tax on coal export profits.
Several Australian states have enacted tough laws against climate protests, drawing criticism from civil rights groups and the United Nations.
Meanwhile, Australia, a major coal producer, is planning new coal, oil, and gas projects.
ss/sri (AFP, dpa)