Andrew,
Since you have already accumulated 80 hours flight experience I have revised my advice which is once again forwarded for your consideration.
The RA-Aus Operations Manual requires Flight Instructor candidates to have accumulated not less than 75 hours PIC with passenger, cross country and radio endorsements before undertaking the in-flight examination. Therefore, I consider that should also be sufficient for a ground instructor applicant who wishes to teach a RA-Aus theory syllabus.
When you have passed the PPL/CPL Aeronautical Knowledge Examinations that will take care of the aeronautical knowledge requirement.
As discussed, the PMI course can be undertaken at numerous organizations.
Lastly and most importantly I recommend that you align yourself with a RA-Aus FTF and obtain direct guidance and supervision from its CFI. This supervision can become in-direct as you gain experience.
Although the Operations Manual doesn’t specifically mention it I’m sure that if the CFI would indicate his or her role in your formal application to the Operations Manager I’m confident it would be successful.
Although I’m not in a position to advise what the Operations Manager will require before he appoints you as a ground instructor, he may require you to demonstrate your competency by giving a lecture to a designated person. In that case make sure you have a lesson plan, follow the syllabus and practise, practise, practise with feedback from the CFI beforehand.
Good luck.