The question asked by the US Immigration and Naturalization Service is "Have you ever been arrested or convicted for an offense or crime involving moral turpitude ...". The question does not ask if you have any currently recorded convictions, or any non-spent convictions. It uses the words "ever been arrested or convicted".
While you may get away with answering untruthfully, lying on an official US government immigration document can have significant and long-lasting consequences.
Entry requirements to the USA are defined by the US government and administered under US law. While the Australian legal system has a concept of spent convictions and allows a judge to find someone guilty, fine them and choose not to record a conviction, the US Government INS will use the US law interpretation of the terms "ever", "arrested or convicted" and "moral turpitude" when determining eligibility.
Which course of action a visitor to the USA takes is their own choice. But if found by the US Government INS to have answered dishonestly, understand there will likely be significant consequences.