The National Broadband Network operator (NBN Co) is warning of a delay of between six and nine months in the rollout of its network using pay TV cables.

Nearly 3 million people are ultimately slated to get the NBN via Hybrid Fibre Coaxial (HFC) connections.

NBN Co said nearly 1 million premises were ready to connect to NBN via HFC and 370,000 had done so already.

However, many of the current connections are plagued by dropouts and slow internet speeds.

When asked by the ABC how many HFC customers had complained about internet dropouts, NBN Co's chief executive Bill Morrow said the number of customers affected was "small, it's miniscule, you won't see it".

Although Mr Morrow acknowledged that not all affected customers would have complained.

"The number of complaints that we see on HFC is higher than what we've seen on a number of other technologies, but the specific number of dropouts is quite small," he said.

"People may be experiencing dropouts out there and haven't reported it or don't observe it enough, because typically you'll find it's in the evening hours that you'll see the dropouts that we're concerned about."

When pressed by the ABC, NBN Co said that 1.1 per cent of HFC connections were affected by dropouts that needed to be fixed.

With about 370,000 homes already connected via HFC, the problems would have hit more than 4,000 households.

Mr Morrow said it wanted to improve those connections first, and then upgrade the remaining HFC infrastructure, before continuing the rest of the rollout.

"This will result in a six to nine month average delay for those people that have yet to connect to the NBN network over HFC," he said.

"And we're going to work through a number of issues focused primarily on those customers that already have the service today."

Opposition communications spokeswoman Michelle Rowland has raised questions about the timing and manner of NBN's announcement.

"We actually had a spill-over [Senate] estimates hearing late last week and this was not mentioned," she said.

"You would have thought that this was something that was being contemplated for some time, this was an important piece of information that NBN Co might want to share with the Senate, but no."

NBN Co said the delay should not affect its plan to connect 8 million users to the network by 2020.