The terms on which workers are being engaged to build the ultrafast broadband network have long been a source of contention with unions.

The employment practices of some Chorus' contractors and subcontractors will be investigated by the Ministry of Business, Innovation and Employment.

Labour Inspectorate national manager Stu Lumsden said the investigations were ongoing and it would not make further comment.

The investigations follow years of angst between the union E tu, Chorus and its contractors over the pay and employment conditions of linesmen building the ultrafast broadband network and maintaining Chorus' copper network.

Many of the workers are immigrants working under layers of contractors.

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Careers NZ estimates "line mechanics" in the telecommunications industry with between one and five year's experience can expect to earn between $32,000 and $60,000 – the lowest pay band among the jobs it lists in the ICT sector.

IT helpdesk staff can usually expect to earn between $45,000 and $60,000, Careers NZ says.

Tensions have bubbled to a head over the last few weeks after it was reported three Indians had been ​engaged on one-month unpaid contracts by a company sub-contracted to Universal Communications Group, Chorus' service company in Nelson.

E tu union organiser Joe Gallagher said it had been seeking an investigation for months.

"Our understanding is the news of this inquiry has seriously rattled Chorus which has instructed its contractors and subcontractors to make sure their house is in order," he said.

An inquiry would need to ensure workers were to able to voice their concerns confidentially, without fear of reprisals, he said.

"Chorus has said any labour abuses involving its contractors are isolated cases.

"We think it's the tip of the iceberg."

Ensuring workers could speak confidentially was the "only way to find out just how widespread this exploitation is, and to protect the jobs of these vulnerable workers", he said.

Chorus spokesman Nathan Beaumont said the ministry had informed it of the investigation of "some of our service companies".

"We have always said that we would co-operate with any investigation and if it shows up anything then it will be dealt with swiftly," he said.

Chorus had been "very clear" that the use of volunteers was completely unacceptable, he said.

"Our contracts clearly require service companies to comply with the law, and include other obligations around reputation, behaviours, presentations and exploitation of workers. If the investigation finds any breaches with any of our service companies then that would also be in breach of their contract with Chorus," he said.