I believe the Government of South Australia is unjust!?

Your talking about basic tax rates which isn't the story at all. Yes, basic tax rates are indexed by income levels, which means that two people earning the same income are in the same tax bracket. But that's the BEGINNING, not the END. Your parents have so many deductions because of their family it substantially reduces their tax load. The single man ends up paying much more because he doesn't have your parents' deductions. If you're upset, ask your parents to tell you about their deductions.