Now obsolete, this was Australian slang for residents of Victoria, the southern-most mainland state of Australia. The Rio Grande in this case is the Murray River which is the border between Victoria and New South Wales.
This term was especially popular in the late 80's/early 90's when Victoria was virtually bankrupted by an incompetent state government, in other words we were poor folk from south of the border. Thousands of Victorians migrated to the northern states.
Things improved from the mid 90's and this term has pretty much died out.
Queenslander: "Bloody Mexicans moving in all over the place. Next thing they'll want to bring daylight saving with them!"
This entry won't work unless I use Mexican (Australia) in a sentence.