A union for migrant workers says Chorus sub-contractors have been taking their employees' cash for promises of visas.
An investigation by the Labour Inspectorate found 73 sub-contractors working on Chorus's ultra-fast broadband network in Auckland failed to keep employment records, pay the minimum wage and provide employment agreements.
The investigation only confirmed what First Union's general secretary Dennis Maga said he already knew - that Chorus sub-contractors had been exploiting migrant workers for years.
He said workers were in a vulnerable position because they were desperate to get work visas but they're too scared to confront their employees about their unfair and illegal treatment.
"They've been receiving promises from some of the sub-contractors that if they ever accept the working arrangement such as cashing out some money from their salary and giving it back to the employer then they'll be able to receive a sponsorship and extend their work visa in New Zealand," Mr Maga said.
The investigation has so far focussed only on Auckland, and there are several more phases before it will be completed, but Mr Maga believes the problem is nation-wide.
He says the government needed to toughen up on all sub-contractors.
Last year the government announced it would double the number of labour inspectors by 2020 in a bid to catch dodgy employers but the Minister for Workplace Relations Iain Lees Galloway said more needs to be done.
"The Government can actually achieve a lot through its procurement practices.
"What this also demonstrates is that a government that doesn't have its eye on the ball can miss some really outrageous exploitation of workers in New Zealand and we have to bring that sort of thing to an end," he said.
Mr Lees Galloway says immigrant workers have been let down by the government-led ultra fast broadband project, which started in Auckland in 2011.
But he said Chorus also had to be held to account.
"Principle contractors like Chorus must pay much closer attention to what's going on in their supply chains," Mr Lees Galloway said.
"It's not acceptable to say it's that this was out of their line sight, that they weren't aware of it, because ultimately they do have responsibility for those people who are working in their supply chains."
Chorus has launched its own independent review into the inspectorate's claims and has asked former deputy state services commissioner, Doug Martin, to lead it.
Chorus chief executive Kate McKenzie said the company was prepared to take responsibility for the problem.
"Obviously nobody wants to preside over an environment where there are allegations of workers being mistreated.
"That's not consistent with our values, that's not who we are so if we find that anything has occurred we will definitely be taking whatever action is required," Ms McKenzie said.
Labour Inspectorate national manager Stu Lumsden backed the review, but believed Chorus should never have had to do it in the first place.
"What we're saying to employers is you have a responsibility to comply with employment standards, don't leave it to a situation where the inspectorate comes along, finds the errors and then embarrasses your brand in the public arena."
Mr Lumsden said once the Ministry of Business Innovation and Employment had finished its investigation into the 73 sub contractors in Auckland, it would look closely at Chorus' contractors throughout New Zealand.