Unfortunately, regulation to protect against monopolies and oligopolies is one thing that our government has not done particularly well. Government regulations exist to protect the consumer and other businesses against unfair practices. Sadly, too often ASIC and the ACCC are toothless tigers.
Other areas are providing services to rural areas where it just isn't econimcally viable for private industry, and protecting against tragedy of the commons (another area where the government has failed too often - Murray Darling system, failed environmental regulations, etc).
Securitisation and financial markets are prime examples of where government regulation is required to protect against predatory practices and risk taking. Lessons learnt from the Great Depression which took 50 - 60 years to unlearn before the final repeal of the Glass-Stegall act.
Regarding government ownership, governments owning national infrastructure makes sense to me, even if they can collect a "rent" for its use. This may include highways, national rail lines, power distribution, telecomm networks, etc. The retail or wholesale arms, though, can and perhaps should be privatised. There is no reason why private industry infrastructure should not exist and compete with government provided infrastructure, providing it is econimically sound to do so and the government isn't funding the private industry, but the there's no way private industry should be allowed to have a monopoly over critical infrastructure (such as Telstra's physical network). There's also no reason why a lot of infrastructure can't be government owned but privately run (on tenders) to avoid gross public service ineffeciencies.
The trick is not to have the government regulate, dominate or subsidise a market which will result in lack of competition or innovation. Like aiding the rent seeking robber baron retailers like Solly Lew, Gerry Harvery and Bernie Brookes. Or subsidise a crap local car making industry, resulting in a lack of innovation. The straight-six engines that we produced locally for the shitty Falcadore cars, for example, were a prime example of lack of local innovation and not even bothering to try to improve quality through research thanks to government grants subsidising the industry. This means that eventually international innovation will highlight just how bad our product really is, making it uncommercial even with the government subsidy - which is what eventuated.
I have digressed to the point where we arrive at Australia's embarassing lack of innovation and research and manufacturing. The Scandinavian countries have companies like Nokia, Saab, Volvo, Ikea, and more. The Dutch have Philips, etc. Hell, even NZ has Fisher and Paykel. These countries are all much smaller than we are. And we have nothing.