The first race was held in 1861 ( see list of Melbourne Cup winners ) over two miles ( about 3,218 metres ) , but was shortened to 3,200 metres ( 2.0 mi ) in 1972 when Australia adopted the metric system .
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 .