As guessed, I do not have time to clear immigration etc. to check in a new bag that way. My transit is only 2 hours, and I'm aware that this isn't enough time to go landside. This is why I thought it might be possible to buy what I want at Heathrow and then gatecheck a bag through to Sydney before getting on the flight to Hong Kong so that I don't need to worry about the liquids rule from Hong Kong to Australia.

I'm familiar with the situation with liquids flying to Australia with a transit in Hong Kong. But I figured I'd try to help my father out, because I didn't even think to tell him of the issue before he flew and I'm feeling a bit guilty.