Not sure about JCU but at Swinburne they offer 16 astronomy units, There is no specific stream for Grad Cert, Grad Dip or Msc. However there are prerequisite units that you must have already completed to enroll in some particular units.
Regardless of the units you choose, it is almost certain that there will be other student enrolled in them that are part way through their Grad Cert, Grad Dip or Msc, there most definitely is not a separate set of units for each stream.
I found that to be a most stimulating experience because you are communicating with other students at quite different stages as they progress toward whichever degree they are enrolled in.
I have attached a link to the Swinburne units.
http://astronomy.swin.edu.au/sao/students/units.xml
If you are interested then it may pay to have a look on Swinburne's site at the thesis titles for the current PhD students, also there is a listing for what Swinburne graduates are now doing, which is interesting.
I have personal experience at Swinburne and I recommend their courses, just as I am sure Renormalized can inform you about the regime at JCU, as he has experience with it. IIS has a broad spectrum of members with experience in astro education so I am sure you will get some good advice from other IIS'ers.
Regards
Trevor