可否雇用本地律師負責外國(澳洲)的樓宇買賣?

The answer is probably no. This is because solicitors in Hong Kong are only allowed to advise on Hong Kong law and therefore are unable to advise and handle property transactions in Australia (even if such solicitor is qualified to practice in Australia, s/he cannot advise you in HK as to property transactions in Australia as s/he as a qualified lawyer in Australia cannot, for example, give usual undertakings to the purchaser in Australia because s/he is not employed by a firm in Australia). You may however consider retaining a Hong Kong firm with Australian connection and see how they can assist you in this respect which may be more affordable than retaining a law firm in Australia yourself. Examples of Australian firms in Hong Kong include Paul Hastings, Mallesons and Baker & McKenzie.