Because 'form' and 'nation' are perhaps non-specific words, there's a few different answers here. Firstly, here's an answer on why convicts were sent to Australia from the UK in the first place. Secondly, Australia was a somewhat varied set of British colonies (not all of which involved convict transportation) until 1901, which is the point at which it became, officially, a nation. In-between 1788 and 1901, many free settlers played a role in the colonisation of Australia; in 1828, forty years after arrival, about a third of the British colonies were free settlers or born in the colony, and by the 1850s, free settlers outnumbered convicts or former convicts (as I explain here). I describe the process of the 1901 federation here, briefly.