I have a vague recollection of school history telling me that Victoria in Australia (or some part of it) was "traded" with the locals for some small token of goods, and that the bargain was made because an escaped convict had joined with the locals and learned the language and could therefore act as translator. Is this still considered to be the case? Who was that convict? And just how dodgy was this bargain?
The deal you're talking about is famous because it is the only treaty ever made with Indigenous Australians during invasion, and because it relates to the founding of Melbourne.
John Batman, a land owner in Tasmania, knew that there was fertile land and good pasture across the Bass Strait, as many settlers (like the Henty family) had illegally set up in what was then an unknown coastline, between the authorities of New South Wales (mostly just Sydney) and Van Dieman's Land (Tasmania). Batman led a group over to the area that is now Melbourne in 1835 and made a deal, wherein he traded cheap but useful goods in exchange for a land lease with the Wurundjeri/Woiwurrung people of the Kulin nation. He split the land with another group of settlers led by John Pascoe Fawkner.
There's some evidence that Batman might have had good intentions, as many settlers saw their own actions as invasion of Aboriginal land, and many felt guilty, but it never stopped them seeking their fortunes from it. After the founding of Melbourne, as disease and dispossession destroyed the Kulin people, he handed out free jam and rice and pitied them. However, he had also tried to purchase this land from NSW in 1827, many years earlier, and was rejected, so his treaty could be seen as nothing but an attempt at circumnavigating government authority by recognising Indigenous sovereignty and then buying it the rightful owners.
Governor Bourke in Sydney nullified this treaty soon after - Australia was supposedly terra nullius ('belonging to no-one'), and the Indigenous people were supposed to have no sovereignty, and supposedly British subjects. Thus, they could not make a treaty or sell land that according to the NSW government was Crown Land.
I've never read anything about a convict translator, but he did take seven Aboriginal men from near Sydney - these men may have been the actual signatories to the treaty. Even if Kulin elders did sign the treaty, they would likely have no idea what it meant beyond being polite, a part of a gift-giving ceremony, and its extremely unlikely that they would have agreed to sell their homeland, since country is sacred to Indigenous people. The land owns the people, not the people own the land, and the people have a sacred duty to care for it.
I'm sorry that there is so little detail here. What I've written here is essentially what 9/10 general Aussie history books will tell you, maybe a bit more, but you'd likely find more detail in a Melbourne or Victoria-centric history book. I'm Western Australian, and I tend to focus on Perth.
The reference the OP makes from memory about an escaped convict is to William Buckley. William Buckley was a 6ft 6in ex soldier who had been convicted of theft and transported for life. In 1803 he was with a party from the ship Calcutta in Port Phillip when he absconded with two other prisoners. The prisoners separated (nothing is known of the fate of the other two) and Buckley eventually met up with local aborigines, with whom he lived, married and had a daughter. He lived among the aborigines, mainly around the Bellarine Peninsula for approximately 32 years when he surrendered himself to a party under John Wedge from a visiting ship in 1835. Wedge considered him likely to be a useful intermediary with the aborigines and obtained a pardon for him from the Governor of NSW. John Batman did employ Buckley as an interpreter on a salary of 50 pounds, however while Buckley facilitated Batman's negotiations it would not be correct to say that he was the reason the negotiations took place