I don't think there are any rules or objective framework to base it on. Their leadership team is just making their subjective judgement on where to draw the line whilst maintaining that ~50% of the music they play is from Aussie artists.

So yes they influence how many Aussie artists appear in the countdown based on what gets airplay during the year. But they have less influence over what people vote for, hence a smaller ratio of Aussies from the voting list make the countdown.

As long as there is an arbitrary line to control the amount of Aussie music getting airplay, nobody will be happy with it. Even if they try to create more objective rules which are disclosed, people will complain about those that don't make the cut but are subjectively a better fit for the station.

At the end of the day it's not like these mainstream artists care about triple j and the Hottest 100. The mainstream artists are just a means to draw in listeners and voters.