As the roll-out of the network accelerates and the speeds available come under growing scrutiny, Prime Minister Malcolm Turnbull said on Monday the project may have been a "mistake" - at least as envisaged by the former Labor government - and admitted there were questions over whether the giant infrastructure project would ever make back the money invested by taxpayers.

But former prime minister Kevin Rudd defended his Labor government's original vision for fibre to the premises in an appearance on ABC's 7.30 program.

"What we had planned and began to roll out was perfectly designed for this nation's needs," he said.

"News Ltd did not want the National Broadband Network, News did not want fibre to the premises, and the reason they didn't want that is because it would provide direct competition to the Foxtel cable network in this country."

The Liberal Party had, he said, "mysteriously" adopted the same position as that company.

Mr Turnbull and Communications Minister Mitch Fifield both pushed back at suggestions from NBN Co chief executive Bill Morrow that a levy may need to be applied to new ultra-fast mobile broadband connections so the NBN can make a profit, as it is losing money on typical connections.

Customers who connect to the internet via a fixed-line connection that is a competitor to the NBN network will soon have to pay a levy of $7.09 a month to help subsidise the NBN, which is required to offer connections to hard-to-reach customers.

Mr Sims told Fairfax Media the concept of protecting government-owned infrastructure from competition - such as through a mobile broadband levy - to help it make money at the expense of consumers was unacceptable.

"We would be totally opposed to it."

He said a comparable dynamic had existed in the energy market where state governments had doctored rules to protect their electricity distribution companies and that "whenever governments invest in infrastructure they have done it on the basis that it succeeds or fails on its own weight, if it turns out not to work then the shareholders, that is the taxpayer, should wear it".

Mr Sims said if a government business was not performing in the way that it should then it should have its value written-down as a private enterprise would but he could not say if the NBN was in that position.

On Monday, ahead of a Four Corners report into the delivery of the network, Mr Turnbull said Labor's original fibre to the home plan was hugely expensive and billions of dollars had been wasted.

"So having been left in a bad place by Labor, what we are doing is ensuring we deliver it as quickly and cost-effectively as possible but I have to say to you, again, one complaint is one complaint too many." Under the Coalition, the roll-out has moved to a multi-technology mix of fibre to the node, with slower copper and hybrid fibre cable used for connections to homes.

Mr Turnbull said he was aware that a lot of people were not getting what they had paid for and was in discussions with NBN Co and the ACCC about the issue. About 4000 monitors will be put in place across the country to monitor speed and network performance.

"At the moment, it [the NBN] is estimated to deliver a return of around 3 per cent, which is not - it is enough to keep it on the government's balance sheet, as a government asset, but it certainly is not a commercial return that the stock market would expect. It was supposed to be 6 per cent," he said.

Complaints about the NBN increased by 160 per cent in the past year, according to the Telecommunications Industry Ombudsman, with more and more users revealing they are not getting the speeds they have paid for and, in some cases, are receiving slower speeds than those they achieved on older ADSL connections, which are being phased out.

NBN Co has blamed retail service providers, who sell connections to the network to the public, for the slower than expected speeds and argued retailers have not purchased enough bandwidth to deliver promised speeds.

Senator Fifield ducked questions on Monday about whether the government would have to take the project back onto the budget as an expense, given the low rate of return. At present the network is considered "off budget" for accounting purposes as it is an investment supposed to make a commercial rate of return.

"The government cannot unilaterally revalue the NBN," Senator Fifield said.

He also confirmed the government was still committed – as Labor is – to sell the network once the project has been completed.

Labor communications spokeswoman Michelle Rowland was sharply critical of the government's handling of the project, but could not say whether in government it would spend billions more to deliver fibre to the home connections or what it would do to fix speed issues.