Newshub can reveal four Indonesian welders working at a Napier sawmill have been getting paid little more than $3 an hour.

It's not illegal; in fact, our current visa rules allow it.

The welders have been putting in the hard yards - working at Napier Pine, where they sleep on-site in a converted shipping container.

"We deserve to be treated exactly like the Kiwi worker. We are hard workers," Santoso, one of the welders, says.

"I receive a monthly salary of 8 million Indonesian Rupiah. But the Indonesian boss tells me that if the New Zealand boss asks, just tell him I earn 9.5 million rupiah."

He says that's because his Indonesian employer keeps part of his pay.

Considering Santoso's timesheets, eight million rupiah works out to be a bit over $3 an hour.

The men's passports were taken off them on arrival, and they say they've never seen an employment contract.

The current immigration rules mean they don't need one.

"During this time, we never, ever have a contract or agreement," Santoso says.

Yukis, another worker, says he's been working under similar arrangements since 2005 and often works late into the night.

"I work long hours sometimes, for example, from 8am in the morning to 3am the next morning."

Dr Glenn Simmons from University of Auckland Business School says that is concerning.

"It's dangerous to be doing that at night with minimum health and safety standards. It does raise some questions about whether they ought to be working at 3am in the morning."

The men came here on what's called a "specific purpose" work visa to install specialised machinery. It's normally a short-term visa.

But they've been here nine months and Newshub has video of the men hammering in foundations and even driving forklifts around the site.

"We don't have the licence to drive the forklift, but because we can ... we just do it," Santoso says.

Newshub's investigation has now prompted Immigration New Zealand to review how it issues "specific purpose" visas for foreign workers, but says because the men were here to install special equipment they don't need to be paid the minimum wage.

"They're employed by the offshore employer, not ours. So we can't dictate the employment conditions or the wages that they're earning," assistant general manager of Visa Services Geoff Scott says.

"If they are doing work outside of that specific work, then that is normally not acceptable."

Napier Pine managing director Mr Mukti also said the underpayment was not his company's responsibility.

"We just give them some pocket money here if they want to buy something. But the company that sends them is the one that pays them."

So Napier Pine technically hasn't done anything wrong in terms of underpayment, but researcher Dr Glenn Simmons, who's met the men, says he's concerned their passports were taken.

"Normally they're retained simply as a control mechanism because individuals won't move far from where their pass books are, even if they don't possess them," he says.

But Napier Pine says they were only held for security reasons.

"So they don't lose it, that's all. That's all, that's all," the managing director says.

In a statement given to Newshub, Mr Mukti says he's a fair man and pays a fair price for a fair day's work.

The men have now returned home to Indonesia.

But Mr Mukti says he's now decided that he will ensure the men do get paid the minimum wage.

If you have further information about worker exploitation contact Michael Morrah: michaelmorrah@mediaworks.co.nz

Newshub.