Workplace health and safety enforcement comes under state jurisdiction, but they must adhere to national standards when they're in place. Typically, any policies regarding scuba fall under workplace health and safety, ergo, the states set their own policies regarding scuba diving while adhering to any national standard(s). Queensland is notoriously strict (the mandatory snorkel rule that we all like to mock), but having lived there for almost a decade, I do actually appreciate why they're so strict. Lots of people who can't even swim want to go to the GBR and learn to dive. Trying to minimize reputational damage to the broader tourism industry by enforcing what may seem like OTT diving rules.
When I said you can do the bubblemaker type courses at 8-10, that remains true, even in Queensland. But they are not certification courses. They're try-out dives. Even in Queensland you can do this at 8 years old. At 10 years old they will let you do Discover Scuba intro dives. You can enrol in Junior Open Water at 12. Technically. Individual dive shops may set their own stricter policies beyond this. My old LDS wouldn't take you out on their day trip boat if you were under 16.