Not even Australian citizens are guaranteed that they'll get a flight back, and hotel quarantine (at least in NSW) is now user-pays, and they're limiting how many people enter the country.
Unless a vaccine appears, I'd be estimating at late 2021-2022 for the international borders to open up again for the rest of the world apart from New Zealand and Pacific Islands, who'll be allowed in long before that. Even if you could get here in September, most state borders are closed*, and the Indian Pacific isn't running. Try and arrange for credits on your trip, because it'll be an amazing trip. I did the Indian Pacific from Perth to Sydney this time last year and it was the best trip ever. Don't expect much sleep when you're in NSW (the rails are rough because of all the freight trains), but I found drinking two glasses of port before bed knocked me out. And on the Nullabor, the tracks are smooth and it's easy to sleep, apart from them waking you up Way Too Early O'Clock to get out and have some sort of outback breakfast at Rawlinna.
* Just popped back to edit this and instead of starting a new thread in GD asking a stupid question, just asking you. Are there any state borders closed in the US? If the NSW-VIC border wasn't closed right now, I'd be sweating it a bit...
Here's some pics of my Sydney-Perth-via-Adelaide-and-back-to-Canberra-via-Sydney trip. I really hope you do get to come over when things have improved and you have a great holiday 
Sorry the pics are so big but I don't know how to make them small...
At a ghost town called Cook in the middle of the Nullabor. I was there in the 80's when it was a town that had a school and shops and a population, but with a poplation of about 4, it's only reason for existing now is to be an emergency base if there was a train disaster:
This is Perth before we got on the train:
And Rottnest Island. It was awesome. If you get a chance to go there, go. Do the trip from Perth and not Fremantle because the river is really pretty on the way out. Quokkas are flea-ridden and nasty animals, though. Don't let them trick you into feeding them..
And this is a random photo from Glenelg in Adelaide after a long day of trying to find my hotel
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