There are claims the cost of the $43 billion National Broadband Network (NBN) could double.

Experienced industry consultant and project manager Malcolm McKenzie has told Radio National's Background Briefing program the costs of big telecommunication rollouts always blow out.

The claim comes as the Federal Government prepares to announce that more than 300,000 extra premises will be connected to the NBN than originally planned.

"I would say for any project this size, [a cost blowout of] 50 per cent to 100 per cent would not be unrealistic," Mr McKenzie said.

He says red tape and rain are just two issues that will cause delays.

But NBN chief Mike Quigley says the Tasmanian trial, which is about to be launched, came in on time and on budget.

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

He says the NBN's business plan shows the network will turn a profit. The business plan is not public.

Mr Quigley says it uses more pessimistic assumptions than a major consultant's report commissioned by the Federal Government.

"What I would say, and I'm not going to go into details of those differences in the assumptions other than to say we came to the same general conclusion that an acceptable return could be generated for the government," he said.

"Our business case demonstrates that quite clearly and our assumptions are probably in some cases more conservative."

Coverage maps

Meanwhile, NBN coverage maps released by the Government show the network will provide fibre optic coverage to 93 per cent of residences, up from its original estimate of 90 per cent.

Communications Minister Stephen Conroy says the extra coverage can be achieved within budget.

"Every home will be connected to the National Broadband Network," he told AM.

"There will be the 93 per cent fibre to the home footprint. There will be the 4 per cent wireless, the next generation of wireless footprint and the remaining 3 per cent will be the satellite footprint."

Regional centres including Weipa, Tennant Creek, Port Augusta, Albany, Cooma, and Warrnambool will be included in the fibre optic network, with the remaining 7 per cent of residences connected through wireless and satellite networks.

Residents serviced by the fibre optic network will have access to internet speeds of 100 megabits per second, while those on wireless and satellite services will receive connections speeds of 12 megabits per second.

At present Australia's average internet speed in 2.6 megabits per second.

The NBN represents a major point of difference between the two major parties in the election campaign, with the Federal Opposition planning on scrapping the network if elected.

Opposition communications spokesman Tony Smith has accused the Government of reckless spending.

"This is $43 billion of taxpayers' money that the Government's prepared to punt," he told AM.

"We'll be outlining our plan in the not too distant future but we won't be spending anything like Labor."

The network is expected to be completed by 2018, with centres connected as the project progresses.

Background Briefing's report can be heard on Sunday on Radio National after the news at 9:00am.