Mr.Qiao Zhou, please don't worry, you can still join Australia nationality at a later time. The consular official just want to make sure that you have not joined Australian nationality When you apply for Chinese passport, China government doesn't recognize dual nationality, if you have joined Australia nationality, China embassy/consulate can't issue you a Chinese passport.

the Declaration of Having Not Joined Australia Nationality doesn't mean that you can't join Australian nationality forever. Now, you can fill and sign it to get a Chinese passport, at a later time, you can still join Australia nationality, then you will automatically lose China nationality.