Is roaming free in Hong Kong if I have a UK (O2, EE, Vodafone) or Australian SIM card?

No, unfortunately, this is another common misconception. Although Hong Kong has historical ties to the UK, it is an independent Special Administrative Region, and it is NOT part of the UK or Australia's domestic network coverage. Most UK and Australian carriers (especially post-Brexit for UK providers) have excluded Hong Kong from their free-roaming zones. You will almost certainly be charged expensive international roaming rates or forced into a costly 'Day Pass' by your home carrier. EastESIM offers a much more transparent and economical way to stay connected in Hong Kong.