Around 20 senior citizens sent Queensland's government headquarters into lockdown with a protest against the Adani coal mine.
The protesters were demanding the government prosecute Adani for an alleged breach of its pollution licence by releasing sludge into wetlands near the Great Barrier Reef during Cyclone Debbie last year.
The protesters entered the foyer with hidden banners at around 11am on Monday.
Ten police negotiated with the group and they quietly left the building around 12.30pm.
Galilee Blockade spokesman Ben Pennings says the group met with a group of top public servants and discussed the suspected breach soon after the protest.
"They were in strong agreeance with us that Adani and all facilities need to be ready for storms and cyclones," he told AAP.
"Adani obviously wasn't (ready) because of the release of the toxic coal sludge."
He said the government had confirmed it was still investigating Adani and would announce the outcome before the September 7 deadline.
Liberal National Party Opposition Leader Deb Frecklington said the government should investigate whether any breach had been covered up, but said the project needed to proceed.
"We know that the environmental approvals are all in place and Adani needs to get going so that those jobs they've promised for regional Queensland start," she said.
"In relation to any prosecutions, that's up to the government to monitor and carry out as they should."
Environment and Great Barrier Reef Minister Leeanne Enoch's office has been contacted for comment.
