It's definitely coal dust. What I think has happened is that the crew has been cleaning the deck and has been interrupted to prepare for the helicopter. They will then get the hoses and brooms out again and continue. It just goes over the side after the ship has cleared port limits (and before it enters the Great Barrier reef zone).

Cheers,
tropic

Coal dust for sure! Are they gonna flush it away or sweep back in hold?

Are those (black) replacement hatches ? Or is it the coal dust again?