Can I renew my HKSAR passport which already expired in Melbourne?

Chan, I think you need visit Chinese consulate in Melbourne or contact them to confirm if you can apply for it. According to Chinese embassy in Australia, Hong Kong people can submit their passport application to the embassy or consulate. The contact information of Chinese consulate in Melbourne.
Address: 534 Toorak Road, VIC 3142　　
Tel: 03－98220604　　
Fax: 03－98246340　　
Email: consulate_melbournemfa.gov.cn