This is the part of the Chinese Nationality Law that is specifically applied to HK and related to your case.
I am curious - did you renounce Chinese nationality when you took up Australian nationality?
Explanations of Some Questions by the Standing Committee of the National People’s Congress Concerning the Implementation of the Nationality Law of the People’s Republic of China in the Hong Kong Special Administrative Region
(Adopted at the 19th Session of the Standing Committee of the 8th National People’s Congress on 15 May 1996)
Where a Hong Kong resident is of Chinese descent and was born in the Chinese territories (including Hong Kong), or where a person satisfies the criteria laid down in the Nationality Law of the People’s Republic of China for having Chinese nationality, he is a Chinese national.