Greenpeace's largest ship could disrupt coal shipments and launch other actions on the Great Barrier Reef over the next week.
The global environmental group's MV Esperanza is docked in Brisbane ahead of a week-long fact-finding mission on the reef, which begins on Sunday.
The ship plans to document threats to the reef, from port developments and climate change, at Gladstone Harbour, Hay Point, Abbot Point and Townsville.
But Greenpeace isn't ruling out the ship taking part in other activities as well.
Spokeswoman Louise Matthiesson was asked if the ship would be launching any protest actions, like blockading ports or boarding other ships.
"It's something we think is a very important tactic for drawing attention to environmental damage and environmental problems and for intervening when we see something that is simply wrong and unacceptable," she told reporters.
"But we don't announce our plans in advance, so I can't give you any specifics about this particular trip."
A journalist then said: "So you're not ruling it out?"
"As I said, Greenpeace has a proud history of using civil disobedience, peaceful protest, where there are environmental controversies," Ms Matthiesson replied.
