Newdegate Cave, within the Hastings Caves State Reserve in Tasmania's south-east, thanks to @laurenepbath. Hastings Cave is Australia’s largest developed cave, and being formed from dolomite, it’s also one of the rarest. And the Marakoopa Cave near Mole Creek in the north-west has the largest glow-worm display in the country. (Tassie’s endearing features are more than skin deep!)
Richmond Bridge, Tasmania. Richmond Bridge (1823). Originally named Bigge's Bridge, Richmond Bridge is Australia's oldest bridge still in use. It was built by convicts from sandstone quarried at Butchers Hill and hauled by hand carts to the bridge site.