Mrs.Sita, you can extend your stay in HK by applying for a visa, and also you can apply for one in advance through CVASC in Australia if you work/reside there.

You can go directly to Hong Kong Immigration Tower at Wanchai with supporting documents enclosed such as return tickets,hotel voucher and papers if any that may support why you need to stay in Hong Kong longer. Probably you can only get a week,maximum of 2 weeks depending on your purpose in HK.