Tensions between the head of the NBN Co board and the Opposition's broadband spokesman Malcolm Turnbull have spilled over.

Letters leaked to the Australian Financial Review reveal NBN Co chair Siobhan McKenna wrote to Mr Turnbull saying company directors had hired lawyers in the face of Coalition "threats" of a judicial inquiry.

Mr Turnbull told the ABC's Lateline program that the Coalition would order a "forensic audit" of the company if elected to government.

"We have made no threats, we have a policy which proposes there to be a number of inquiries, a number of exercises, a strategic review, an audit, and a cost benefit analysis," he said.

Ms McKenna told the Financial Review that the NBN is a "politicised environment and people don't always play the ball".

But Mr Turnbull says the statement was inappropriate.

"The only question that I've raised is this: Why is the NBN Co paying for a political lobbying firm to promote the abilities and accomplishments of the chair and her directors to, among other people, members of the opposition," he said.

Subcontractors say they are owed $15 million

About 30 subcontractors working on the rollout of the national broadband network in Tasmania say they are owed up to $15 million.

David Mier from the Communications, Electrical and Plumbing Union travelled to Hobart last night to meet one of the subcontractors hired by Visionstream.

He says some subcontractors have not been paid for four months.

"This is putting these peoples' financial viability and their employees viability at risk," he said.

"We may well have a scenario where there will be 500 people laid off on the NBN project because their employer is not getting paid. It's an outrageous scenario.

"There's no suggestion as yet that there will be industrial action in Tasmania, it's certainly being discussed but sub-contractors need to be paid."

Visionstream says some work has been delayed because of asbestos issues and it is working with subcontractors on schedules and pay rates.

Meanwhile, 25 subcontractors working on the rollout in Ballarat in Victoria are still off the job because of a pay dispute.

Staff from two subcontracting firms said on Monday they would not be returning to work amid complaints of drip-feed payments and disagreements about contractual terms.

One firm told the ABC it would not be sending its workers back on the job until the dispute with the contractor, Transfield, is resolved.