Grad Cert is a post grad qualification. As is Masters. In the first course I did, the Grad Cert was the first half of the Masters. The Masters I finished was 120 units though, so it only gave me credit for 1/3 of it. That course is now only 80 units for Masters.
In case you’re wondering, I got Distinctions and High Distinctions for the Grad Cert (the standard wasn’t really post-grad in my opinion). The Masters I completed had more academic rigour and I mostly got credits.
A lot of Uni and technical college/ vocational training courses have different exit points. Sometimes it can be cheaper to do a lower level course and use it to get credit or advanced standing in higher level courses.