It sounds like the will just combine it (which can only be a good thing). You got a decent STAT score, and that alone would get you in, now you have a UAI to add to it too. It can only make it better.
How will the decision be made on whether an offer to study at Macquarie is given?
STAT results will be included in a selection process which also considers your ATAR and any additional study.
I had no UAI at all and still got in on STAT alone.
Trust me, you’ll be fine. It’s only the wait that’s the horrible part.