AC is the highest Honour Australia currently bestows. Until 1986, there was the AK - Knight of the Order of Australia - which made it's recipient a Sir, but the Labor government of the day ended that Honour.
Australians can still be awarded Knighthoods by the Queen, which would make them a Sir or a Dame, but such an award would be made through the Imperial Orders not through the Order of Australia - there is speculation that John Howard might eventually receive such an Honour.
Likewise, the Queen could, in theory, grant an Australian a Peerage which would make that person a Lord or a Lady, but this has occurred only a handful of times in Australian history, and is very unlikely to ever happen again.
I hope some day PM Howard get that honours I doubt it under that stupid Gordie Brown adminisation maybe in Tory party govt he may get it maybe David Cameron would suggest it