NBN Co’s long-term profitability will be restricted if the Coalition’s proposed mix of cable, copper and fibre networks is not replaced with a full fibre network in coming years, government briefings reveal.

A draft confidential NBN Co report, prepared for the incoming government during the caretaker period and obtained by Fairfax Media, warned that it was "essential" for the Coalition to factor the cost of upgrading to a fibre-to-the-premises network into its financial forecasts.

NBN Co's long-term profitability will be restricted without a full fibre network in coming years. Credit:iStock

The absence of high-capacity services on a fibre-to-the-node network would compromise NBN Co’s ability to achieve the 3.5 per cent growth in average revenue per user target set by the Coalition in its April 2013 broadband policy.

The revenue hit would be potentially so problematic for the Coalition’s policy, the NBN Co report warned, that the government should start planning for its own full-fibre rollout in its financial modelling.