My hong kong passport is already expired, can i renew it in hong kong or i need to renew it in sydney?

You can apply from the HK Immigration Department directly or from the Chinese embassy in Australia. If you apply from the embassy, it usually takes four to eight weeks. The cost is around 40 Australian dollars. If you hold old HK ID card or you never apply for it, please apply for the new HK ID card when you apply for the new passport. You should provide a recent passport size color photo in white background, a copy of your HK ID card, filled ID843 form and other documents required by the authorities.