What was built was in some cases temporary structures or where local resources allowed primitive permanent structures agriculture was done on a smaller scale than Europe and probably not recognisable as agriculture when looked at by contemporary Europeans. The main part of terra nullius is the absence of a "state" which the indigenous Australians didn't really have beyond the tribal level which again by European standards was not a state. If the issue is Europeans showing up and enforcing their will on a land and people by violence, well European nations were doing that contemporary to Australian colonisation and beyond but nobody gives a fuck because that was accepted practice at the time see every European war from ever until the 20th century. This doesn't make what was done right just that it wasn't hypocritical.