A friend got her citizenship in Jan 07 and went back to the UK to show the family her baby (born 9 days before ceremony). She got on the Cathay flight with UK passport no problem. Stopped in Hong Kong to visit friends and on checking in for Brissie was stopped for Aus passport. She didn't have one but they just rang DIAC who confirmed she was an Aus citizen and was allowed to board the flight. Took all of 30 minutes, even with the language barrier.

Not advisable, and she was probably lucky but it can be done.