God knows how we got approved to do the approach?
I'm not God, but I'll take a wild guess at the answer to that. It probably had a bit to do with training, certification, maybe even some periodic proficiency checks, and procedures in your Ops Manual that talk about action in the event of non-availability of GPS.
If that isn't enough and if, as I suspect, you're in Oz and using TSO C129a gear, then you need an alternate that has a serviceable navaid. But then, I'm sure you knew all of that already.
