The federal government has thrown more than $20 billion into NBN Co to get it up and running.
Commonwealth figures value NBN at $29.6 billion but the Parliamentary Budget Office has put a price tag of just $8.7 billion on the utility.
Finance Department heads on Friday told a parliamentary inquiry into the network that the difference represented a loss to the taxpayer.
"NBN is in a start-up phase of its business and that is what you would normally expect in a start-up phase," first assistant secretary Tracey Carroll said.
"You would take the equity that was given to the business and then there are higher levels of expenditure until the business turns into a profitable business."
NBN Co is a government-owned entity established to oversee the national broadband network.
It currently owes the Finance Department nearly $20 billion which the department expects to recoup by 2024, having pushed back the original 2021 deadline.