The cost of rolling out and maintaining the national broadband network has risen to $57 billion, senators have been told.

Completing the rollout had a budget of $51 billion, but with NBN Co’s new investments another $6 billion will be spent.

Department of Infrastructure, Transport, Regional Development and Communications assistant secretary Andrew Madsen said the rollout had finished.

“It’s now moved into its operating phase, where it is continuing to connect new premises to the network – as it would be expected to do,” he told a Senate estimates hearing on Wednesday.

“It’s also made the decision, in conjunction with the government, to invest in additional network investments that will provide extra capacity for business and residential premises, as well as other enhancements across the company.”

He said the investments were expected to generate a return.

Then-communications minister Malcolm Turnbull had promised to cap the government’s equity investment in NBN Co to $29.5 billion.