Not to forestall further answers, but my answer to the question Why was Australia colonized? What motivated people to travel so far only to settle in such a dangerous place? might help answer your question. I've also answered a question about when the numbers of free settlers arriving in Australia started to exceed the numbers of convicts, and another about the doctrine of terra nullius that led to the convict colony being in Australia rather than in Africa. (In general, scholars seem to think that the British would have gone about colonising Australia very differently if their real motivation was because of things like military strategic importance.)