If you go by length instead of ridership it seems extremely plausible. Australia is a big place and building 2 lanes for mostly freight trains is an unnecessary expense.
Sidings/loops/passing tracks, allow trains to pass on a single track, and since things can be more regular scheduled (ideally anyway) with trains, they can work well enough and with much less track building needed.
If you go by length instead of ridership it seems extremely plausible. Australia is a big place and building 2 lanes for mostly freight trains is an unnecessary expense.
Sidings/loops/passing tracks, allow trains to pass on a single track, and since things can be more regular scheduled (ideally anyway) with trains, they can work well enough and with much less track building needed.