Original: The race has been held since 1861 (see list of Melbourne Cup winners) and was originally held over two miles (about 3,218 metres) but following preparation for Australia's adoption of the metric system in the 1970s, the current race distance of 3,200 metres was established in 1972.
Simplified: The race was first held over two miles, about 3,218 metres, but after Australia changed to the metric system in 1972 it was changed to 3,200 metres.