Many people in this area will have seen large grey-brown moths appear after the wonderful rain we have experienced. About the size of a tennis ball (in diameter) they are Swift Moths of the Family Hepialidae. The young larvae of these moths (also known as “Witchetty or Bardi grubs”) nibble surface vegetation before burrowing into the soil where they feed on roots (grasses, eucalypts). The genus shown is Abantiades*. The caterpillars pupate into a honey coloured casing from which the adult moth emerges, usually after late summer or autumn rain. There is an excellent summary of information at http://en.wikipedia.org/wiki/Witchetty_grub.
* Note that I orginally misidentified this as a Goat Moth (Family Cossidae). Many thanks to Mike Halsey for picking up the error (see comment).