Walking around the garden recently, I came across this mating pair of Cairns Birdwings, Ornithoptera euphorion.
They were hanging in the shade of a silver wattle which has Aristolochia, their caterpillars’ food plant, sprawling over it, so their location is unsurprising.
The fresh empty chrysalis nearby and the not-quite-developed wings of the female suggest that she had just emerged and the male had seized the opportunity to mate her before her wings were fully dry and she was ready to fly off. That also is unsurprising, since it happens often amongst butterflies; the females emerge as fully mature adults, so they are never too young in the way that (e.g.) mammals or reptiles might be too young.