Hi Chris
As Brendon pointed out it will depend upon the school and their Physics teacher. In Year 11 VCE Physics there are six detailed studies of from which two are studied. One in each semester. Astronomy and Astrophysics are two of the detailed studies. Since its introduction into the VCE study design over ten years ago I have regularly taught the Astronomy detailed study at Year 11. To the best of my knowledge there is no Cert IV Astronomy course. The Year 10 National Curriculum also has some Astrophysics but you'll need to check with your local school as to how much time they give to it - often it gets glossed over fairly quickly.
http://www.australiancurriculum.edu....tent-structure
There some Citizen Science projects in Astronomy that can be found online that may be worth considering.
HTH
Paul