Each state legislation is different. Check your local rules to see what is required. I know from my dealings with the Air Force Cadets we often get issues in that activities interstate may require staff to get that states check as well. For example if I was to travel to QLD I think I can do 7 days with my WA WWC Card but anymore than that I need a blue card.
It might have changed a bit now but I am not sure. TAS and SA didn't have an equivilant for a while there (possibly still not).
In WA it is anyone that deals with a person under 18 unless you are a parent of a participant acting as a volunteer (or something similar to that - there are other exceptions).
Best advice is google your state and find out from the govt of the day what the requirements are.