Recently went through the flinders & Oodnadatta on a set of pretty worn (5-6mm) 19"ATR's, covered 5500km in 9 days. Dropped the pressures down to around 28-30psi when on the dirt and didn't suffer at all, the tyres were barely scuffed.
I've just bought a new set, 3rd set I've had. Unless it was heavy mud or very rocky I'd be comfortable using them.

