JonClarke - That's exactly what I was looking for, thank you so much.
That should convince my aquaintance and his friends - unless he wants to start believing in a secret underwater civilization living in Australian rivers

Coincidentally, I worked very near to Kununurra in '73/4 when I was an Assistant Surveyor for the Commonwealth Water Resources board, based in Darwin. We spent most of our time in the bush, looking for water all over the Northern Territory. (And drinking vast quantities of beer.)