I visited the Portland silos many years ago, the guys there then said the big shed holds 500,000 tons and each of the 24 concrete bins holds 2000tons, these at the time I am speaking if were used for all sorts of grains (oats, barley, millet etc) but NOT wheat
They stated ships usually just topped up at Portland because the channel out of Geelong was not deep enough for the ships to take a full load, where as the channel into the grain loading dock at Portland is(was?) 55 ft deep.
THe guys there then provided me with an excellent tour, was VERY interesting. The only place I was not shown was in the tunnel where the grain trains unloaded, because the dust down there was chokingly thick.