The airlines are responsible for paying return flights for passengers that are refused entry. Therefore they are the ones who often enforce that round-trip ticket rule
so that they don't lose out. Most of the time 1-way international tickets are actually more expensive than round-trip ones; I have never found a cheap
1-way to the USA. So you might have to bite the bullet and get a round-trip ticket to appease the airline and just not use the return leg. Or if you get a good one-way ticket to the USA then go and buy a (very, very expensive) round-trip ticket from the USA back to Oz that is fully refundable and get your refund after entering the US. This ticket is most likely going to have to be full-fare and run around 3-5K$ Australian.
It only took me a couple of days to get my multiple re-entry visa in Sydney
- all it did was cost a pretty penny.