Virgin Australia space to Australia is phenomenal. Last week I was adjusting an award redeemed using SkyMiles and there were at least three biz award seats available on LAX-BNE. LAX-SYD was also available on the date I was looking at. The fuel surcharges are also gone now, so it's a great option. Transfer 40K SPG to 50K DL and you've got the 300K needed. Alternatively, sign up for a DL AmEx if you don't already have one, which should get you a good chunk of the way there.

Another benefit to flying VA (as opposed to KE/CZ/MU via Asia) is that you don't have weird routing restrictions on visiting other places in Australia. For instance, you could fly ATL-LAX-SYD(stop)-AYQ, take the train or some other transport to ADL, maybe drive to MEL, and then return MEL-(SYD/BNE if needed)-LAX-ATL. You're going to have trouble finding fares with suitable routing rules to get to AYQ (or any number of places beyond SYD, MEL, and BNE) in Australia via ICN/CAN/PVG.