To be honest, given that you plan on getting married, you should take it really easy with visa-free visits, because it will be extremely easy for the CBP to assume you plan on simply remaining illegally.
I personally would hardly recommend more than a single one-month visit within a 6-month period. Definitely bring a return flight confirmation and proof of ties to Australia.
Then, when the time comes, if you plan on settling in the US, get a derivative E3-D visa
That said: yes, the clock resets when you leave the WHTI area (basically North America+Caribbean), and so you could be granted a new 90-day visa-free stay in the US - at the CBP's discretion of course
Also your ESTA (which is not a visa, but an online passport registration needed for most people except Canadians to enter without a visa by air or sea) is valid for 2 years unless your passport expires sooner. So no need to apply twice even if visiting twice.