Delays for millions of future NBN customers expecting to connect through their existing pay television or internet cables could cost taxpayers up to $790 million, a statement from Labor claims.

The delay of new customers being added to the hybrid fibre coaxial (HFC) network for six to nine months, could cost from $423 million to $790 million, according to a joint announcement from shadow minister for communications Michelle Rowland and shadow minister for finance Jim Chalmers.

Labor claims the delay of the NBN's roll out of HFC connected homes could cost up to $790 million. Credit:Glenn Hunt

They pointed to the 2016 NBN Corporate Plan, which included a sensitivity analysis for a situation where delays occurred - and a revenue per home of $47 a month.

But the government has questioned the accuracy of the figures - saying the NBN was still working through the financial forecasts, and the estimates in the Plan were made before the HFC network was being rolled out.