Details are being held tightly. Cost blowouts associated with the complexity of the project and delays as NBN Co renegotiated Telstra's role last year are expected, however, and they will rekindle political debate about the government's decision to replace Labor's fibre to the home network with a hybrid network that includes fibre to the home, fibre to the neighbourhood node, and hybrid fibre that was laid two decades ago by Telstra and Optus. The Coalition launched its multi-technology NBN plan in April 2013, and said then that it would cap government funding for the project at $29.5 billion. It confirmed the cap after it was elected, and is expected to maintain it when the new cost estimates are revealed. It will therefore be able to state that the cost blowout will not result in taxpayers paying more money to build the broadband network. Holding the equity cap at $29.5 billion as the cost of the rollout rises means however that NBN Co is going to have to fund more of the project on its own balance sheet. Debt was expected to be the big source of additional funding when the cost estimate was $41 billion. If the new cost estimate is significantly higher, NBN Co may have to consider innovative funding techniques, including private equity. For the government meanwhile, higher project funding costs inject life into an argument it is not interested in having: whether or not it is getting value for money out of a network redesign that trades operating speed for a lower build cost, and faster delivery. The new report is expected to conclude that some of the forces that are increasing the cost of the Coalition's multi-technology NBN rollout would also have pushed up on the cost of Labor's rollout, which the NBN estimated in November 2013 would have been $73 billion.

The government will therefore still be able to argue that its NBN project is cheaper than the Labor project it scrapped. Whether that overcomes the potentially problematic politics of a cost blowout for its version of the NBN project remains to be seen, however. The government is under pressure over policy project management, and it is its NBN project that is live. The new numbers will lengthen a trail of estimates and guesstimates about how much Australia's broadband network is going to cost by the time it is built. In April 2013 when Tony Abbott and Malcolm Turnbull announced the Coalition's version of the NBN, they said it could be built more quickly than Labor's network, at an expected cost of $20.4 billion. NBN co was estimating at that time that the peak funding cost for Labor's fibre to the home network would be $44 billion. In an opinion piece published days after he and Abbott launched the Coalition's plan Turnbull said however that Labor's project was "a shambles" and "likely to cost $94 billion."

The Coalition won power in September 2013, restructured NBN Co's board, and told it undertake a strategic review. The review came out in December with an estimate that a multi-technology rollout would have a peak funding cost of $41 billion, well above the Coalition's original $20.4 billion estimate and the $29.5 billion equity cap. The strategic review also concluded however that Labor's fibre to the home network would have cost $73 billion if it had gone ahead – below Turnbull's earlier $94 billion guesstimate, but well above NBN Co's previous estimate for the Labor government that a fibre to the home network would cost $44 billion. The multi-technology rollout could therefore save more than $31 billion, NBN Co's new chairman, Ziggy Switkowski, said. Those two project cost benchmarks were maintained by default last November, when an updated corporate plan was released. NBN Co was still renegotiating its deal for Telstra to co-operate with the broadband project at the time. With that crucial contract not yet reconfigured for the multi-technology rollout and the details of the complicated hybrid network still unclear NBN Co was "not in a position to generate forecasts with a reasonable level of confidence for FY2016 and FY2017", the corporate plan stated.

NBN made what it called "assumptions on possible outcomes" for the two years in the plan. It also cut and pasted the 2013 Strategic Review's estimate that the entire project would cost $41 billion, with $29.5 billion of it coming from the government. It's a fair bet that everyone involved had a good idea last November that the cost estimates were already too low, in part because the negotiations with Telstra had taken about six months longer than expected. The deal with Telstra was finally inked in December, lighting the fuse on new cost estimates that will surface on Monday.