The federal government's investment in NBN Co to date has seen the Commonwealth's net worth drop by $8.8 billion, the parliamentary budget office has revealed.
In its report on the national broadband network's impact on the budget, the PBO revealed the government's investment in the NBN had cost it $580 million each year for the last two years.
That figure is set to rise to $730 million in 2019-20 and $2.1 billion by 2026-27, assuming the government's new $19.5 billion loan to NBN Co is drawn down from next year and paid back in full by 2020-21.
The government's investment in the NBN to date - around $20 billion as at June 30 out of its $29.5 billion in equity funding - was responsible for an $8.8 billion deteoriation in the Commonwealth's net worth, the PBO said.
It blamed the drop on public debt interest (PDI) payments on the government's equity and debt contributions to NBN Co, as well as NBN Co's accumulative losses of $8.3 billion, and $1 billion asset revaluation stemming from "the early years of the rollout of the NBN".
It said the government's decision to loan NBN Co the remaining $19.5 billion it needs to finish the rollout - at an interest rate above the government's cost of borrowing - provided a "partial offset" to its annual costs.
The PBO said it had no idea when NBN Co was expected to start paying dividends to the Commonwealth, leaving it unable to predict when the network builder will have a positive impact on the underlying cash balance.
"The final cost of the Commonwealth’s financing of NBN Co will not be known until NBN Co is privatised and the market places a value on the NBN," the PBO said.
"Until then the Commonwealth will continue to bear an annual cost associated with its financing of NBN Co.
"If the sale price of NBN Co is less than the Commonwealth’s cumulative cost of financing NBN Co, then the NBN would have an enduring cost to the budget."
NBN Co's value at June 30 was $13.1 billion, the PBO said.
The government's $19.5 billion loan to NBN Co will involve monthly drawdowns totalling $10 billion next year, $6.6 billion the following year, $2.5 billion in 2019-20, and $400 million in 2020-21.
It is expecting the principal of its loan to be repaid in full by 2020-21.