The $37 billion National Broadband Network could be delayed by up to 10 years, according to a senior Melbourne academic.

On Thursday NBN Co downgraded its mid-year forecast for the rollout by at least 34 per cent, blaming the delay on contractors.

It has dropped its June 30 target of 341,000 premises connected to 190,000-220,000 premises, but the company maintains it is on track to complete the network in 2021 as planned.

Senior engineering lecturer at RMIT University, Mark Gregory, says he expects NBN Co to downgrade its targets even further before the end of the year, which would have flow-on effects for the overall project's timing.

"If we continue down the path that we are going with external contractors doing the rollout, we'd expect [the rollout] could take five to 10 years longer than predicted," Dr Gregory said.

"We should expect it to cost anywhere between 50 to 100 per cent more than before."

Dr Gregory says there is evidence that NBN Co contractors are "cherry-picking" the areas they connect.

"Certainly I believe that we are seeing it happening already with the figures that we are getting from NBN Co," he said.

"The contractors are missing difficult areas, the rollout itself has already been planned as such that only areas that are easy to do are being cherry-picked."

But Dr Gregory says the problems could be fixed if the NBN Co goes ahead with creating its own construction arm to build the network.

Melbourne property developer David Steele has two land developments where homeowners are waiting to be connected.

He said he was initially told by NBN Co the network would be running past the developments by January this year.

Instead that has been pushed back to May.

"I'm not confident it will be even connected by then either," Mr Steele said.

Poor timing?

The NBN Co said its downgrade only worked out to around three months of a decade-long project and it was confident it could recoup the delay.

The cutting of targets was announced during a phone conference arranged with around an hour's notice - right in the middle of Labor's leadership crisis in Canberra.

During the call, NBN Co CEO Mike Quigley denied any link and said he could only release the figures after the board met earlier that day.

"I can assure you that the events taking place today were of no connection at all to the process that we're going through," Mr Quigley said.

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Opposition communications spokesman Malcolm Turnbull is dubious.

"It was pathetic really, it's a very old and shabby and cynical media management technique," Mr Turnbull said.

Mr Turnbull claims the board knew there would be a delay in the rollout weeks ago.

"The board was aware of this failure in the rollout... for quite a few weeks. So they've known about this for a long time and they frankly should have made those figures available immediately," he said.

"Now if they'd been a publically listed company they would have had to make them available immediately. So this is another one of those examples where a government-owned business has less accountability than a publically listed company."

NBN Co blamed the cutting of targets on its contractors.

This week it took over responsibility for the construction in the Northern Territory from contractor Syntheo.

Syntheo will continue to manage the rollout in Western Australia and South Australia.

"We are accountable for the delay and are disappointed it as occurred," Mr Quigley said.

A spokesman for Communications Minister Stephen Conroy said there were short term challenges in a 10-year infrastructure project which would be overcome.