That's true! But pigeons usually stay at home, don't they? Only the racing pigeons fly so far and they're trying to get home.
I've read it's far more common that racing pigeons from China show up in Australia. I presume all those are killed too. I daresay we get a few from New Zealand too.
I think if a bird is banded, remove it to quarantine as soon as possible, then contact the owner and invite them to choose between quarantinng then release, returning home on the next available flight, or culling, with a deadline on the decision eg one week. If the owner does not respond, cull the bird unless others have offered to cover the cost of its quarantine.