It would be nice if it was an equal balance across the scales through, Tom. Almost everything is HO, or HOn3.5 for narrow gauge because approximately 35% of Australia's railways are 1067mm, or 3ft 6in gauge correct as of 2014 (the other 65% or so being Standard, Broad (5ft 3in), and other types of Narrow (2ft and 2ft 6in). The trend started here in Queensland, which was actually the first place in the entire world to adopt a narrow gauge for a mainline railway network. Ultimately it came down to cost, and a narrow gauge using 30lbs rail with an extremely light loading gauge was all the government could afford at the time. Some lines were even built without ballast.
Unfortunately the range and quality of items available in N Scale are expensive, come in kit form, or are of average quality. This was what led me to model the BNSF instead. I hope this changes one day, though.
Mitch