While I understand that the United Kingdom and China have a treaty with each other regarding the handover of Hong Kong (related question), as far as I'm aware, Australia was not a signatory to such a treaty. While Australia was a former colony of the UK, and is a member of the Commonwealth, I can hardly see Australia being legally obliged to honour the conditions of a treaty signed by the UK in 1984.
Hong Kong has a separate immigration system from China, and holders of Hong Kong passports are already treated differently from holders of ordinary PRC passports, so it's not as if Australia is creating a new division between China and Hong Kong, as if Australia were proclaiming the existence of a place called East Turkestan.
Yet there's reports that Australia's offer to HK residents is likely to infuriate China.
What is China's objection to Australia's plans? I'm interested in both official rationales, and any unofficial reasons.