Alexandria is what they call a Business Hub (or use to be, might have changed again by now). The one tonne vans for that part of town operate out of that facility, and it is substantial depot, but it is a primary collection centre for high volume of van traffic. Generalised consolidation of the freight occurs here, and is then sent on trucks to Chullora, the BIG depot for sorting and interstate truck linehaul to it's destination. Express Post gets diverted into the formerly known as StarTrack Express Air Courier Network in different depots and takes a more direct logistic journey to airport centric depots for distribution.
Basically the biggest depots can't cope with the high traffic from thousands of small vans in each city every day. Freight is consolidated in the suburbs and then trucked to the big depots for bigger trucking or flying out of the state.
Same in reverse, freight coming into the city arrives at the big house, is sorted to suburb by delivery van run, then sent out on trucks to the parcel centres (formerly known as business hubs) and it all starts again, every day.