The true cost of the National Broadband Network to taxpayers has been detailed for the first time, with a new report showing the project has wiped $8.8 billion off the federal government's balance sheet since it began.

The independent Parliamentary Budget Office (PBO) on Wednesday released a report on the budget impact of the NBN, which warned it could have an "enduring" impact on taxpayers if the government cannot sell it off at a good price.

The NBN is considered a government asset and is run "off budget". This means its short-term impact on government finances is not detailed in the budget papers like other programs.

The PBO report shows the government's investment in NBN Co has resulted in an $8.8 billion deterioration in the government's net worth, as of June 2016.