The government is withholding a report on the NBN so its progress can't be scrutinised by the public ahead of the election, the opposition says.
Opposition communications spokesman Malcolm Turnbull made the claim during a debate on Monday night against Deputy Prime Minister Anthony Albanese on the ABC's Lateline program.
"Anthony is sitting on the latest revision of the business plan and has not released it to the public," Mr Turnbull said.
"You've made them keep the draft stamp on it so you don't have to give it out before the election.
"You've got a confession from the (NBN) company that they're failing and you don't want to let the public know."
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Mr Albanese said he hadn't received the document and denied the project was failing.
He stood by the government's plan to connect 8.5 million premises to fibre by 2021 at a cost of $37.4 billion.
But Mr Turnbull rubbished the figures, saying the government's plan would cost $94 billion compared to the coalition's $29.5 billion.
The coalition would use "the technology that delivers the service you need in the most timely and cost effective way," he said.
By using existing copper networks, which can provide download speeds of 100 megabits per second to homes within 400 metres of a fibre node, "three-quarters or more of the construction cost" could be saved, Mr Turnbull said.
But Mr Albanese said the coalition's figures were "plucked out of a Coco Pops packet".
"This is an absolute nonsense with no basis in fact," he said.
Labor's NBN, which would deliver download speeds of 1000 megabits per second and upload speeds of 400 megabits per second, would "transform the way that education, health and aged care services are delivered," Mr Albanese said.
And while speed was important, reliability in the network was crucial, he added.
"That's why fibre is important," he said.
