Dear Sir/Madam

I was born in Hong Kong, and as a child had moved to Australia with my family in early 1996. I am an Australian citizen, and have not returned to my place of birth since I came here. I'm planning to return to HK to spend Christmas there later this year, and my parents have recommended that I obtain a HK ID card before my trip. How do I go by this? What are the costs? Is it crucial that I procure a HK ID card for a 10 day holiday? What are the advantages of this?

Thank you