The last time Australia held a general election as well as a referendum on the same day was 1974. Plenty of referendums have been held. The last time was in 1999.
I have systematically checked the dates. The years were 1906, 1910, 1919, 1928, 1930, 1946, and 1974. This suggests that maybe they had a few problems, because it hasn't been done for a quarter of a century.