Wouldn't the police - and the indigenous tracker - have been able to tell if there was earth movement there?
I think the ground is likely pretty solid in that area. Which is why it is used for sheep grazing and has not been turned into agricultural land.
It is not easy to dig into our outback soil. It has been baked under a thousand suns. And apparently eroded by an inland sea in days long past.
The pic I saw of the sandpit (which I have been looking for, and can't find again) looks like it was pretty close to the house, surrounded on three sides by bushes/trees (likely for shade and wind protection), and quite shallow.
And I think the sand would have been placed on top of the hard outback ground, not actually a pit.
imo