What I think would actually be the case is something similar to the
USA Land Rushes of OTL. Basically the UK sets up a Commission to charge a certain amount for the right to partake in the Land Rush. At a designated spot those who have paid have the right to run to the designated lots to claim one. The interesting thing about doing this in Greater and Lesser Australia is that the UK and the Rail companies would be working together. I see it something like this.
The UK government needs rail lines built to supply their forts along the UK/French border in G. Australia. Cheap labor can be found in using the prisoners of the penal colonies as chain gangs. As many of the chain gangs work as little as possible, and violence only gets them to work so far, it is announced that those who work to completing the lines will accrue an amount of money they can use to purchase their own land upon their release.
This works so well that the British government and the rail companies decide to do something similar with anyone who wants to buy their own land. The Rail companies setup outpost/towns, in which they own all the buildings and goods. Those that have purchased the rights to a land grab obviously need to buy their temporary room and board, supplies, and livestock from the Rail Road. The UK government gets their cut also.
Thinking about it more the mining companies started in Australia earlier than the Rail companies, so I could see them getting in on this also.