Malcolm Turnbull, the opposition spokesman on communications, said that the "extraordinary" cost of gaining access to the NBN will limit its number of users.

The opposition has not yet finalized its policy for broadband. However, Turnbull indicated that the Coalition is not likely to change its stand on rural broadband. He earlier questioned the government's claims that the project would provide benefits to consumers and competition.

Retailers are likely to be charged about $35 a month for NBN Co's services. Turnbull said that customers are likely to pay anywhere from $65 to $70 a month.

In an interview with the Sydney Morning Herald, Turnbull said, ''that is higher than most people are paying now. So there is no reason to believe that the NBN will deliver cheaper broadband. It certainly will deliver faster broadband than many people are getting at the moment, but at an extraordinary cost.''

Retail prices for the service range from $30 to $160 a month at network test sites in Tasmania.

Turnbull said that the government's monopoly could weaken competition from other technologies. He also questioned the government's claims that the project would increase competition between retailers.

A spokeswoman for Stephen Conroy, the Minister for Communications, pointed to Tasmania as proof that increased competition will happen. ''As experience in other markets has shown, the introduction of genuine competition will lead to more choice, more affordable prices and higher quality services.''

NBN Co, which is government-owned, is expected to release details of its business case soon.