This picture is taken in the early 1900s at the Wyndham prison. Wyndam is the oldest and northernmost town in the Kimberley region of Western Australia.
It was established in 1886 as a result of a gold rush at Halls Creek. However the circumstances and the story behind this picture remain unknown. The Aboriginals could have been arrested under the various local laws passed that forbid them from entering or being within a certain distance of named towns.
They could also have been arrested for drinking or owning firearms which was illegal for them at various times. It's also possible that they have been rounded up to be moved to a reserved areas which were being created at the time that these individuals did not want to move. It could even be a staged picture for tourists/publicity reasons.