well vagrancy is a tricky business. It isn't that a hypothetical bird just comes down as soon as it sees land (e.g. the north coast of Australia) and then carries on later to Adelaide. Instead Adelaide is the place it first lands. You can say that certain points of a country are the most-likely spots for vagrants to arrive, but through their very nature of being vagrants even remote spots can also be land-falls.
Migrating birds often use airstreams, which are high-altitude wind currents enabling the birds to travel longer distances at less energy expense. A lost bird can just keep on going in an airstream because it doesn't know where it is, until it basically falls out of the sky. It may be over the ocean or it may be over land. A good example is a willy wagtail (for overseas people, actually a fantail not a wagtail) from the Chatham Islands east of NZ. It was obviously a vagrant from Australia, but it completely bypassed NZ (which it literally had to fly over) and ended up on a little island in the middle of the ocean. That individual is the only willy wagtail ever recorded from the NZ region.
With birds like waders and ducks, they often (not solely) become vagrants because an individual accidentally gets caught up in the migration flocks of other species. For passerines vagrant to Australia and NZ it is usually just a mistake of that individual.
