Check this out:
http://www.nationaltrail.com.au/
http://www.nationaltrail.com.au/bikes.html
It's 3312 miles long, whereas the Great Divide is 2,711 miles (including the Canadian portion). Altho it looks like the route is most popular with horseback riders, MTBs are allowed, but the site says some areas are so difficult a rider might need to hit the pavement to ride around the rough areas.
I'm planning on riding the Great Divide soon, I think this trail might have to follow! What do you think?