How do Hong Kong residents' children apply for foreign relatives' adoption?

First of all, you should consult your immigration office about the procedures and conditions of adoption. Secondly, it depends on whether your sister is Australian International. If so, after meeting the conditions, be sure to ask my sister to consult the local civil affairs department in Australia to see if local laws can be applied for adoption. If there is no local international, she is also a Hong Kong citizen and only needs to comply with the laws of Hong Kong.