From the Qantas terms and conditions effective March 31st, 2011.
10.7 Smart, casual dress standards apply at all times. Individual lounge managers will have discretion to administer these standards as they reasonably deem appropriate in the circumstances.
If Qantas wants to use thongs as an excuse to keep a female sex worker, known in my days of use and in the politest possible word available in the dictionary as a prostitute, from entering the lounges then well done to the airline. It's bad enough having the places littered with half cut public servants, politicians, diplomats and business travellers on expense accounts without the admission of pimps, whores and charlatans.