Took me a fair while to identify this butterfly but was pleased when I did. Think it is more prevalent further north from here, in the SE queensland region. Turns out it is a male Orchard Swallowtail butterfly. There is quite a difference between a male & a female of this species. It apparently lives on citrus trees but its caterpillars cause only minor damage. They feed at night & lie quite still during the day to avoid caterpillar-loving birds. The adult feeds off the nectar from flowers. There is a lemon tree next door so it may have come from there.
They are slow & erratic flyers. They are members of the swallowtail butterfly family but, like some of the other members, do not have this feature. They are large butterflies measuring 10.5cms across.