Not strictly true. The relevant number is population density as lived. Australia is really big with large empty areas, but it has huge concentrations of people on the coasts in urbanized, developed environments.
The NBN still needs to go to where people live, which means stretching the grid out over far more land than in France. The width of the lain cable doesn't change the point that you still need to get someone out to dig a trench in the first place, as it were... France is only about 700km wide - that distance barely gets you out of our most populated state if you start from from its capital and drive for the nearest border.
You don't actually have to provide fibre to someone living in Cobar for the NBN to be useful though. If you cover the ten biggest cities in Australia, you have covered about 90% of the population. If you cover the ten biggest cities in france, you haven't even got to 50%
That doesn't really matter though, since the goal is 100% coverage. I mean, technically, the NBN was 'useful' even when it just covered the couple of test-run blocks (one ended about 500m from me...).
I've been a city boy all my life, but we really need to stop treating regional and rural Australia as second-class citizens when it comes to these kinds of things.
The other thing you point out is interesting - the urbanisation rate of Australia. It's in the top 5 'big' countries, sitting at 90% - above France at 85%, and the US/UK at 80%. It kinda goes against the mythical image we have of ourselves, the rugged outback-seeker. Sod the wide brown land, give us tarmac!
