Giving a keynote speech to the Commsday Summit, a two-day meeting of Australian telecom leaders in Sydney, Mr Turnbull will say the Coalition favours a cheaper fibre-to-the-node (FTTN) model, in which broadband fibre ends up to a kilometre from homes, which are then connected to the network by copper wire.

Mr Turnbull says the NBN model, of joining broadband fibre to the home, is three to four times as expensive.

He also says the fixed-line network is being built on an uncertain business model, with internet users increasingly turning to mobile and tablet devices.

''NBN Co has been fairly aggressive in its estimates here - after accounting for empty premises, it only expects about 13 per cent of households and businesses to go mobile-only by 2021,'' he said.

Mr Turnbull will argue that instead of the more expensive NBN model of fibre to the premises (FTTP), the government should invest in FTTN.

Under the FTTN model, data speeds vary depending on how close residents are to the node, and bandwidths are generally lower than the NBN model, which will deliver to-the-home service to 93 per cent of households.

But Mr Turnbull said the government should instead look at connecting homes now in broadband hot spots, saying the revenue generated by customers using high-speed broadband services was ''the great imponderable''. ''Nobody knows what new applications will emerge and what consumers will pay for them,'' he will say.

''To be candid, any Coalition broadband plan will face similar uncertainties about the level of future demand, which is why deferring expensive and irreversible commitments such as FTTP to the extent that this is possible is simply prudent management, not some bloody-minded refusal to face the future.''