My passport is going to be expired. Which office should I ask for extending my passport?

Where do you live in Australia? The China embassy/consulates in Australia have their own consular districts:
Embassy of China in Canberra: Australian Capital Territory (Canberra District), South Australia, North Territory
Consulate General of China in Brisbane: Queensland
Consulate General of China in Melbourne: Victoria, Tasmania
Consulate General of China in Perth: Western Australia
Consulate General of China in Sydney: New South Wales