This does not appear to be a mistake fare, just a serious fare war. And it’s as cheap as I have ever seen Australia flights. The fares appear available through at least end of November for the outbound flight.
This isn’t a pure mileage run, it’s a great fare to see Australia, since you can’t return until 10 days from your departure to qualify for the fare. Fares are non-refundable and there’s a $300 change fee. So only buy if you’re sure you want the tickets, but act quickly as these fares won’t last long.
Here’s the prices and cities suggested by SecretFlying:
Minneapolis – Melbourne: $534
Seattle – Melbourne: $534
Salt Lake City – Melbourne: $534
Atlanta – Melbourne: $534
Detroit – Melbourne: $535
Minneapolis – Sydney: $542
Seattle – Sydney: $542
Salt Lake City – Sydney: $542
Atlanta – Sydney: $542
Detroit – Sydney: $547
Salt Lake City – Brisbane: $570
Minneapolis – Brisbane: $575
Seattle – Brisbane: $575
Atlanta – Brisbane: $575
Detroit – Brisbane: $610
Here’s a calendar of availability in June for week to 10 day Australia trips, Atlanta – Melbourne:
So these fares are widely available not just on a few days. Here’s what Atlanta – Melbourne looks like:
The fare basis is LLE01BM0. ‘L’ fares earn 100% of flown miles with Singapore Airlines KrisFlyer so I’d consider crediting the flights to a Singapore Airlines account to earn nearly 20,000 miles roundtrip versus about 2000 for a base member of the MileagePlus program.