[Answer below mainly applies to medicine rather than dentistry]
Apologies, I should clarify my statement - you'd still be eligible for JMP and WSU (subject to your UCAT result) and Bond but the rest of the unis would most likely be out - by taking a leave of absence you don't really have anything to gain because the fact that you have a tertiary study record already rules out most Aus unis, and by continuing tertiary study your chances at the unis that do accept non-standard applicants will either a) not change or b) be reduced by doing so.
Once you have a tertiary record of study you're only eligible for schools that accept non-standard applicants. For medicine, these are JCU, WSU, JMP, UNSW and Bond. For JMP, you need a close to credit average GPA (this is just a hurdle requirement) and for WSU, your 99 ATAR also meets the hurdle (getting an interview at these unis will then depend solely on UCAT result). For Bond they'd go off your ATAR (or possibly a combo of your ATAR and uni GPA) to get through to the psychometric testing. You'd still be eligible for these unis if you did what you proposed above as I already mentioned.
The other two (UNSW and JCU) will use your GPA in order to determine interview rankings. If you have less than 1FTE year of study (8 units or equivalent) completed, then your GPA is affected significantly. UNSW use a 50:50 ATAR:GPA combo as part of their selection criteria - they convert your GPA to an ATAR and then find the average of this value and your high school ATAR. See this table below for how we believe UAC coverts GPA to ATAR:
Note how if you've completed less than 1FTE of study, your equivalent ATAR is going to be lower than if you've done 1FTE or more. In your case, the best case scenario would be you achieving a GPA of 6.5 or better in your first semester of uni and then this being converted to a 98 ATAR - combined with your 99 from high school this would give you a 98.5. You'd need a very strong UCAT to offset the lower than average ATAR to make you competitive at UNSW, hence my "less competitive" comment from above.
I believe (not entirely certain, however) QTAC (who rank applicants for JCU, Griffith and UQ for med and dentistry) do something similar for those with a tertiary record. Again, for JCU medicine because you have less than 1FTE year of study, I think this would make your academic rank much lower and you'd need a very strong written application (and possible interview) to get a place as a non-standard applicant (especially if you're non-rural). For Griffith dentistry, you're immediately out of the running as a non-standard if you have less than 1FTE year of study.
I shouldn't comment on other dentistry schools because I'm not entirely familiar with how different universities would treat applicants who do what you are planning to (I know you'd be ineligible for UniMelb, USyd and UWA on account of having a tertiary record and Griffith and JCU due to GPA calculations explained above but can't comment for the rest -
Yamster may be able to help).
Hopefully that clears things up a bit for you.