No, there was no slavery as such of the Aboriginals. From the late 1800's to the 1960's many of the workforce on the outback cattle stations ( ranches )were Aboriginals. Drovers, cooks, cleaners etc and they lived in accommodation provided close to the main house. As the introduction of cattle was detrimental to the traditional hunting grounds, and as the Aboriginals began to lose their hunting skills, they worked to eat. In some cases they were paid in a form of script that could only be converted into food / tobacco etc. at the station store or they were paid with food.
This procedure tended to "lock" several Aboriginal families to the station as effectively as slavery would have done.
As the stockman ( cattle station owner / manager ) is dying he has no further need of their labour and some of them will be as old as he is anyway, past working age.
As far as I'm aware there is no reason to tie kangaroos down unless you want to stop them hopping. I think - Tie me kangaroos down sport - is just a well-imagined lyric.
The word - sport - as used in this context means a good mate or buddy.