Not wanting to prolong this, but the next paragraph gave the full meaning...

"According to Fast Tracks (admittedly a book primarily about car circuits in Australia), motorcycles raced on a longer 12+ mile circuit which included most of the AGP circuit from a date somewhat later in 1928 than the 100 Miles Road Race, which later became renamed as the Australian Grand Prix."

The circuit the Australian Grand Prix for motorcycles has used is just under three miles. It was three miles, but they shortened the inner section a tad because the ground there was always wet and caused it to break up.