Malcolm McKenzie, an experienced industry consultant and project manager, claims that the cost of the $43 billion National Broadband Network (NBN) could double. McKenzie, who was interviewed by Radio National's Background Briefing program, says that the cost of big telecommunication rollouts always gets inflated.

The comments were made with the impending announcement of the government's plan to add more than 300,000 extra premises to the project.

"I would say for any project this size, [a cost blowout of] 50 per cent to 100 per cent would not be unrealistic," McKenzie said, adding that the weather and red tape may cause delays.

Mike Quigley, NBN's chief, has put forth a different claim, saying that the Tasmanian trials have been on time and on budget.

"I would be surprised if, as we move forward, we didn't get in fact better," he said.

Quigley says NBN's business plan predicts a profit for the network, adding that the project used more pessimistic assumptions than a report commissioned by the Federal Government.

Areas covered by the network will have Internet speeds of 100 megabits per second. Wireless and satellite users, on the other hand, will have speeds of 12 megabits per second. The country's current average internet speed is 2.6 megabits per second.

The network may be finished by 2018.