To those from the Western hemisphere, it’s always fascinating to hear that some homes and businesses from the times of the Greek philosophers still have inhabitants, and then you remember that the Western hemisphere is itself not without its own examples, for example some Mexican villages still have temples from the times of the Mayans.
If you’re talking white people, technically cooks cottage (built 30 years before the first fleet) but it was brought over and reassembled in the 1930’s lol
Indigenous, probably the eel traps / farming system. That’s 9,000 years old iirc
Or if you’re up in the top end, there’s Nawarla Gabarnmung which dates to about 44kya. Some natural rock pillars in the shelter, others modified and others still constructed or relocated.