A medical certificate is one of the conditions required to have your instructor sign you off for a solo flight. Also they won't sign you off unless you do part of your training with them as they are required to check that your aeronautical knowledge is up to scratch. However as long as your fly dual, there's no need for you to have a CASA medical certificate. Much depends on how much flying you want to do and how much time (and money) you are willing to dedicate. In my opinion the easiest thing to do is to have a chat with a local school and say that you want to do some casual specific training, e.g. steep descending turns, stalls and spin recovery, etc., while you're in Brisbane. No school that I know of would turn you down.
PS. Where are you doing your PPL?