As you've discovered, the PBS database is publicly available online. I use it all the time.
For all the modified release versions of Methylphenidate, it says you're required to have been diagnosed with ADHD between ages 6-18 to qualify for subsidy as an adult.
For immediate release all it says is it's for ADHD, with no age of diagnosis requirements listed that I can see.
Bluelight has strict rules about discussing drug prices. I would think those rules shouldn't apply to discussion of the legal price obtained via a legitimate doctors prescription at an ordinary chemist, since that's readily available public information and not relevant to or encouraging solicitation of illegal sources or black market prices or availability. But that interpretation of the rule is not for me to decide, so I'm going to err on the side of caution and not directly list any prices and confine my comments to how the system itself works.
So that said, yes, since the PBS subsidizes methylphenidate, if you qualify for a PBS prescription under the authority system, then with a health care card you would qualify for the reduced price just as with any pbs subsidized medicine. And upon reaching the safety net with a health care card, no price whatsoever. General patient charge is the cost to you without a health care card.
But that's if your situation is covered by PBS regulations. And as it says on the pbs database, modified release is only subsidized for adhd if diagnosed as a child. But immediate release might be covered for anyone with ADHD.
Hope any of this information helps.