The rollout of the hybrid-fibre coaxial (HFC) part of the troubled national broadband network is known internally as "Operation Clusterf---", says Labor senator Stephen Conroy.

"I know the internal nickname you've got for the HFC rollout, and it's not fit to actually describe on the public record: Operation Cluster … something," Mr Conroy told NBN chief executive Bill Morrow during a heated exchange at a Senate hearing on Tuesday.

The Turnbull government and the company rolling out its so-called "multi-technology mix" have been under increasing scrutiny over the viability of the HFC part of its network rollout, which uses outdated Telstra and Optus HFC cables (the kind used for receiving Foxtel).

Media leaks last year showed the poor quality of Optus' HFC network and Telstra's copper nodes meant they would cost more than expected to fix up.