Supporting evidence NBN Co is the government-owned corporation building the network. Its latest corporate plan says it will end up needing $44.1 billion, $30 billion of which will come from the government and the rest from borrowings it will do itself. Mr Turnbull says three of the assumptions behind the plan are over-optimistic, and another one looks dicey. The first is that its wholesale earnings per user start at $22 per month and then climb to $62 by 2020-21 when the NBN is finished. Mr Turnbull says that's a dramatic increase of 9 per cent per year above inflation. There are few precedents for such an increase. For a decade now real fixed-line spending per person has been falling. NBN Co is assuming a huge turnaround. Mr Turnbull says if earnings per user climb by a more modest 3.5 per cent per year above inflation the cost of building the NBN will be $8.3 billion higher.

The second assumption is that NBN Co can wire up each fibre connection for an average of $2400 (satellite and wireless connections will cost much more). Mr Turnbull believes a more likely figure is $3600 (which is around what analysis such as those in Macquarie Bank arrive at). He points out NBN Co's finances are ''very sensitive to fibre-to-the-premises costs". The extra expense would amount to $12.9 billion. The third assumption is that the NBN will be finished on time. Due in 2021, it has already missed many deadlines. NBN Co itself describes its task as ''challenging''. By 2016 it plans to be wiring up an extraordinary 6100 premises per day. If the schedule slips so that it finishes four years later, in 2025, the project would cost an extra $14.9 billion. (The assumption Mr Turnbull believes is merely dicey rather than over optimistic is that within a decade 16 per cent of households will be wireless-only and not wanting the NBN. If the figure is a higher 25 per cent the NBN will need an extra $1.9 billion.) Put together and building on each other, these ''quite moderate and realistic'' assumptions push up the total cost of the project to $94 billion by the delayed completion date of 2025. By then Mr Turnbull says NBN Co will also be losing $5 billion per year.

Does it stack up? On earnings per user, NBN Co chief Mike Quigley told Parliament in April that he was making $38 per month, not the $22 quoted by Mr Turnbull (although Mr Turnbull's figure came from Mr Quigley's corporate plan). That meant Mr Quigley wasn't expecting nearly as rapid and unrealistic build up in revenue per customer as Mr Turnbull supposed. But Mr Turnbull's office reckons Mr Quigley's figure is artificially boosted by an overrepresentation of speed-loving early adopters among those who are signing up early. They are prepared to spend more for speed than will later users. On costs, NBN Co has told the parliamentary committee it is managing to connect each user for $2200 to $2500, close to its assumption of $2400. Mr Turnbull believes that estimate excludes some costs and is dominated by the low cost of the easier connections NBN Co is doing first. Eventually it will have to do the harder ones. A counter argument is that costs will come down over time as the contractors discover efficiencies. Several contractors have pulled out of their deals with NBN Co this year saying they are unable to complete the work for the money on offer, suggesting NBN Co may have to pay more than it expects. On the question of delays, Mr Quigley told the committee in April that he stood by his target of finishing the NBN by 2021. Then in July he resigned, declaring his job of laying the foundations for the project ''largely complete''. NBN Co had expected to have 566,000 active users by June 2013. It managed to connect 33,600. Mr Quigley told the committee in April he had ''reprofiled'' the work and expected to have 7500 contractors on the job in June. In August broadband minister Anthony Albanese said that the NBN was employing ''over 4500 people'', suggesting that deadline has not been met. The winding down of the mining construction boom should ease labour shortages although unforeseen developments such as the discovery of asbestos in underground pits could slow progress further. A Citibank report predicts Labor's NBN will finish one year later than forecast.

On the question of households going wireless-only, Mr Albanese's office points out this would also be a problem for the Coalition's cheaper NBN. Finding Mr Albanese told Lateline this week that Mr Turnbull's $94 billion figure had been ''plucked out of a Coco Pops packet''. ''This is an absolute nonsense with no basis in fact,'' he said. ''And indeed, the joint parliamentary committee, of which Malcolm is a member, did consider this and found there was no basis in fact.'' In fact the committee made no such finding. Mr Albanese's office says he misspoke. He meant to say that in giving evidence to the committee the then head of NBN Co Mr Quigley found Mr Turnbull's figure had no basis in fact. Mr Quigley's evidence was nowhere near as definitive.

Mr Turnbull is on strong ground when he says the final cost of Labor's NBN would be likely to exceed the claimed $44.1 billion. Most major projects go over budget. Australia's Parliament House was to cost $220 million. It eventually cost five times as much, $1.1 billion. The Sydney Opera House cost 14 times the original estimate. But the Snowy Mountains Scheme is said to have been delivered on time and on budget. Labor's NBN is bigger than them all. So difficult is it to estimate the cost of such a project that parliamentary budget officer Phil Bowen this week revealed he would not have been able to cost the Coalition's alternative NBN plan had it asked him to. The government says NBN Co's corporate plan has been ''independently verified by KPMG and Greenhill Caliburn'', but the KPMG study wasn't independent (NBN Co paid it for the task) and it took place before construction began. The Greenhill Caliburn review was a paper exercise, which did not examine actual performance. Labor's NBN would end up costing $44.1 billion if a lot of things went right. So far several have not. Turnbull's estimate of $94 billion is unverifiable, but possible if Labor won the election and did not alter its NBN plan. If Mr Turnbull had merely said that it was highly probable Labor's NBN would end up costing more than the claimed $44.1 billion PolitiFact would have rated the claim ''true'' or ''mostly true'' based on what is known.

None of the senior telecommunications analysts PolitiFact spoke to wanted to go on record. Each shared one or more of Mr Turnbull's doubts about the NBN Co's assumptions. Mr Turnbull has gone further and quantified those doubts. His estimate may be on the high side, it may be on the low side. PolitiFact finds Turnbull's claim that Labor's NBN would eventually cost $94 billion possible but unverifiable. It rates it "half true". Details at www.politifact.com.au Loading

Fairfax is partnering with the Pulitzer-prize winning service PolitiFact during the election campaign. Its Australian arm politifact.com.au uses the same rigorous methodology as its US parent to rate the accuracy of claims by elected officials and other influential people in the Australian political debate. Twitter: @1petermartin @PolitiFactOz