If you declined the offer from the main round, then they probably gave it to the next person in line in the late round.
I'm guessing, if there are still vacancies in that course you can always try final round - but final round isn't really a solid thing to rely on at all.
If your solidly above the cut-off for that course, you could probably get a main round offer next year, but really do make sure your above the cut-off, because it could go up for next year. And because your ATAR is still valid if you apply through the UAC next year, then if you haven't done any tertiary education, they will only look at your ATAR. Also remember, that in the case you do re-apply for this course through UAC for next year, you WILL NOT be awarded bonus points for your school subjects. i.e. UNSW offers bonus points which they add to your ATAR for doing well in relevent subjects, but bonus points are only added if your coming fresh out of highschool.
If you do study something else at a tertiary level in the meantime because you want to internally apply for your desired course (the one you declined) or because you want to apply through the UAC again next year but don't want to do nothing for one year, then either way your WAM/GPA comes into play. For internal transfer it would be totally dependent on your WAM/GPA if you get into your course, and for a UAC application they would consider both your WAM/GPA and your ATAR.
However, if you never take tertiary education, then like I said before, UAC will only look at your ATAR when deciding to give you a course or not. The disadvantage to this is that one year is gone and also bonus points vanish.
If you do take tertiary education and then internally transfer of change through the UAC they will look at your ATAR and your WAM/GPA. The disadvantage being that you'll have to work hard enough to have a good enough WAM/GPA.