The only thing I can think of is airlines do "weight restrict" certain flights, and the first thing bumped off is what we call Non-Revenue Passengers, such as buddy pass holders, parents, employees, etc. And if a flight is chronically weight restricted, perhaps they do just embargo non-revs to save themselves the hassle of dealing with them being stranded and pestering them.
This happened to me on Continental Airlines, coming back from Hong Kong. The flight was wide open -- more than 100 seats available -- but I wasn't given a seat. They use any excess weight for cargo, with Hong Kong being a huge cargo hub. So, the load planner in Houston will look at the numbers and say, "Well, we're booked to 200 folks, I need 80,000 lbs of fuel, that leaves me 25,000 left over. And then he okays the flight for 25k of cargo, for example. The only "non-revs" that get on are the ones taking the place of no show passengers.
With Australia being such a long flight, maybe the fuel loads are so high they restrict buddy pass flyers so they can use extra weight for that.
Best option is to call Delta and talk to them. If Reservations can't help you, see if you can get a direct line to the city you want to fly in and out of...they might know. Or ask for their "Pass Bureau..."
Hope this helps...and good luck!
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