An investigation has been launched into companies contracted by lines company Chorus that allegedly used unpaid 'volunteers' or workers paid well below the minimum wage.
The Ministry of Business, Innovation and Employment (MBIE) confirmed that the Labour Inspectorate was investigating the employment practices of some contractors and subcontractors of Chorus.
A Chorus spokesman said the company had always been "very clear that the use of volunteers was completely unacceptable".
"As soon as we were made aware that a few people were taking part in the initiative we put an immediate stop to it," he said.
Chorus had always said it would co-operate with any investigation and should it show anything it would be dealt with swiftly.
Union E tu said in a statement the investigation followed cases of unpaid workers on a "volunteer" scheme run by a Chorus subcontractor, which the company had subsequently put a stop to.
A second case involved a man who was paid $12 an hour - more than 20 per cent less than the minimum wage - by another Chorus subcontractor.
In the latest cases to emerge, migrant cable workers faced multiple breaches of their employment contracts, the statement said.
"For the first few weeks, instead of wages they only received an allowance of about $150 per week. Then, money was deducted from their pay though they weren't told why. They also worked up to 80 hours a week, some of it unpaid, while some weeks there was no work at all."
Chorus had said any labour abuses involving its contractors were isolated cases, but E tu spokesman Joe Gallagher feared this was "the tip of the iceberg".
"We have said before that any inquiry needs to ensure strict confidentiality for any workers prepared to speak out about what's happening. That's the only way to find out just how widespread this exploitation is, and to protect the jobs of these vulnerable workers."