The Abbott government expects to make less money from the NBN because of slower speeds available under the Coalition's copper-based network, a Senate committee has heard.

Facing aggressive questioning from former communications minister Stephen Conroy, government officials admitted that the company building the NBN would have to lower its revenue forecasts, partly because of the cheaper, but inferior, technology preferred by the Coalition.

Slower speeds are expected to hit revenue for the government's NBN. Credit:Peter Braig

There are at least three reasons why the NBN Co expects to make less money from customers on the Coalition's network – which is expected to be about $17 billion cheaper to build than the previous Labor government's NBN.