I am trying to work out the validity of some oft repeated family history. It's been claimed that my grandfather, who served in WWII, witnessed the bombing of either Hiroshima or Nagasaki. I was just wondering if there was any evidence any allied troops, particularly Australians, were in the area at the time.
Absolutely. There were POW camps at both Hiroshima and Nagasaki. Hugh Clarke's Twilight Liberation even contains a first hand account of the Nagasaki explosion from one of the Australian POWs. Of course this assumes your grandfather was a POW. I can't think of any other reason an Australian might be within sight of the explosion otherwise.
If you're interested in trying to track down more info on your grandfather you may want to try mansell.com. The website operators are trying to compile a complete history of Japanese POW camps. They have an impressive list of prisoners, camps, and a lot of original source docs, that may help you piece together your grandfather's time as a POW.