James Matra – who had been a midshipman on Endeavour (under the name of ‘Magra’) and who later became a diplomat – proposed the idea of a colony in New South Wales in August 1783, with the support of Joseph Banks.
The American War of Independence had been under way since 1775, and Matra’s suggestion (which was rejected) was that the colony could become a place for Americans who were loyal to Britain. Matra didn’t mention convicts, but later amended the proposal to include them alongside free settlers.