Apparently not, Am. I'm not across the detail but I possibly there are regulated charges for roaming, which would go nowhere near compensating for the actual cost.
But we don't need to know the legislative detail, we can work the realities out just by thinking it through from a business point of view. The cost of all that high-tech equipment being used by just the handful of people in the outback at any one time is such that even the telco with by far the largest number of customers to amortise the investment over (Telstra) doesn't go anywhere near covering its outback service costs. Adding some small amount from the (relatively tiny) customer base of an Optus or a Vodafail would be a drop in the bucket. (Remember that if you gave (say) 1 million Vodafail customers roaming rights onto the Telstra network, the number who would actually use the outback towers so much as once in any given year might be ... oh ... perhaps a couple of thousand. Even if you charged them an outrageous $10 per call, that's twenty grand - an utterly trivial amount which wouldn't even cover the administrative cost of billing them. (Add a few zeros to my estimate if you like - it's still not enough to make a worthwhile difference.)
Telstra, on the other hand, can spread that cost across their entire customer base. All Telstra customers (no matter whether they live in Balmain or Brunswick or Birdsville) pay the so-called "Telstra tax" which, among other things, goes to fund the outback network. Telstra is by far the most expensive major mobile service provider. How do they get away with it? By having the best network. Without a clear network advantage, Telstra could not charge their 10 or 15 million customers Telstra's usual 10-20% over the going rate. From Telstra's point of view, the millions they lose each year maintaining uneconomic outback services is repaid with interest by the massive marketing advantage it gives them. In short, they would be completely crazy to ever let an Optus or a TPG use their outback towers. All of a sudden, they would have to compete on price instead of quality - and any business that is reduced to competing on price alone is usually doomed to failure.
Put it another way: if the Telstra board decided to give a competitor outback roaming rights, they'd spend the next three years defending class actions from angry shareholders - and almost certainly losing.
Of course, Optus, VHA, and TPG are perfectly entitled to build their own towers if they wish, and they do - but only in places where they can anticipate a reasonable return on their investment. For them, it would be completely useless spending hundreds of millions improving their outback coverage because, in reality, they are not going to go close to equalling the reach of the Telstra network without spending insane amounts. And if they go crazy on spending and started getting anywhere near Telstra, what would stop Telstra putting up another hundred towers just to stay in front of them? Nothing. It would cost a bit, sure, but because of their huge existing advantage, Telstra is always going to be well-placed to win a spending war without even raising a sweat. The boards of the other two (soon to be three) mobile operators know all this, of course, and aren't daft enough to start a war they know they cannot win. (But, as we have just seen, they are bang alongside the idea of persuading the authorities to simply gift it to them. Who wouldn't like to get the use of some other company's very expensive capital equipment without paying for it? Bit of a no-brainer. You'd be nuts not to try it on.)
In short, the current situation cannot and will not change without legislation. Given the number of retired people with large amounts of their savings held in Telstra shares and depending on those fully-franked dividends for their living expenses, it would be a brave government which acted on this. (Look at the flack they took after putting a very, very minor extra tax on banks this year - a tax so small that share prices only dipped for a short while and rapidly recovered.)
Is this a sensible way to run a country's telecommunication network? Of course not. Telecommunications is a natural monopoly and, like all natural monopolies, it works best and stays cheapest when an organisation tasked with operating in the public interest and owned by the entire nation operates it. Unfortunately, we went through a period of economic madness between about 1980 and 2005 or so and gave away our hard-won assets to greedy private interests. The result has been disastrous. Telecommunication got worse and worse and worse until the original NBN effectively renationalised the fixed-line component and restored some sanity. (Far from perfect, and now crippled by cheapskate reversion to outdated copper for most of it, but vastly better than anything on the horizon in the pre-NBN era.) And as for the spectacular failure of a similar sell-it-all-off policy in the electricity sector, nothing I can say is remotely as persuasive as the very large numbers on your latest bill.