NBN Co is almost seven million broadband activations short of its 2020 goal and will need more than the $29.5 billion committed to the project, its annual report says.
Over the 2015/16 financial year there were just under 1.1 million NBN activations - well up on the 485,615 the previous year.
As well there were 2.9 million connections "ready for service".
However, NBN Co has set itself the goal of eight million activations by 2020, which chairman Ziggy Switkowski and chief executive Bill Morrow described in the report as "challenging".
"The challenges should not be underestimated," they said.
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"We remain convinced that the strategy of combining the most appropriate technologies to provide the fastest, most cost-effective rollout of the new network is the right one, but it is a strategy which commits the company to agility and a willingness to pivot fast as new and better processes and technologies emerge."
When the coalition government took power the strategy was changed from focusing on fibre to the premises to a "multi-technology mix" involving fibre to the premises, fibre to the node, hybrid fibre coaxial, fixed wireless and satellite.
Labor has described the project as one of Prime Minister Malcolm Turnbull's most "humiliating stuff-ups" of his first year in office.
The government has so far pumped $20.3 billion into the project, out of a total commitment of $29.5 billion.
The report said "additional funding" would be needed to complete the rollout and NBN was exploring the option of private sector debt funding.
Over the year NBN Co generated total revenue of $421 million, up $257 million on the previous year, but expected a net loss after tax of $2.75 billion.
The report said the main risks in coming years were meeting the "scale challenge", managing partners in the design, construction and activation process and competition, revenue and regulations.
