if the consular officer refuses your 90 days stay duration, he will issue you a single-entry visa permitting 30 days stay duration, you don't need to reapply for the vsia with 30 days stay duration. It is very hard to ensure your 90 days because you can't provide sufficient documents to prove you need to stay in China for 90 days. The round-trip flight tickets (flight ticket from Australia to China, the flight ticket from China to your home country or another country) which can prove your planned stay duration in China.