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 .
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 .