In the U.S. providers in the same practice can talk about clients and its not a confidentiality breach. It's possible the same rule applies in Australia as well. Otherwise they would need a release. If your providers have each other's contact info then I would guess they are either in the same practice or a release with this information was given at some point. Are you a minor? If so a parent or guardian can give consent for this.

Have you told your doctor that you're not allowed to take the medicine? They can't force you to take anything you don't want to, although it may be worth at least talking to them. If anything maybe they can explain why they think medication might benefit you.