Australia is big but most of it is a god-forsaken wasteland where nobody would want to live, like the British Isles. If the French occupy part of the good land, then the territory is extremely restricted.Australia is huge. A British government wanting to have good relations with the restored Bourbon monarchy can give back the French colony and still have plenty of room to establish their own elsewhere. We have this conception of Australia as a single country but that just was the OTL outcome. It could have very easily been partitioned.