We went for another drive this afternoon - along the track leading down from the back of the bluff.
This time we drove off the track a little and stopped to look at a large water tank I'd noticed on other occasions.
On inspection the tank was empty. A small group of zebra finches flew up from an adjacent bush. They perched along the tank edge and then fluttered down into the tank, pecking along the shaded dirt. The little birds weren't worried about us and we were able to observe them quite closely.
There was a small bore beside the tank, so it seemed the tank could be filled, if necessary. Once full, the water could be channelled, via pipes leading from the tank, into a number of long water troughs.