Butterfly

Rapid Bay, Fleurieu Peninsula - any lepidopterologists around who can tell me what this guy is called?

Thanks, Nighthawk3! It is indeed a Wanderer or Monarch (Danaus plexippus plexippus), a species introduced to Australia in the mid-1800s together with the exclusive food plant of its caterpillars, the milkweed family (Asclepiadaceae).
cf.
www.samuseum.sa.gov.au/page/default.asp?site=1&id=1350