A group of Chinese migrant workers helping authorities investigate an illegal migrant scheme are gutted that they have been excluded from the new one-off residence offer.

Some of their fellow colleagues who are not helping the case, have been left free to progress their visas through to full residency.

The Migrant Workers' Association said that's unfair and kicking the workers out of the country risks letting foreign worker trafficking continue unpunished.

It is almost four years since brick-layer Mao Qunyou moved here, returning only once during that time to see his wife and daughter in China.

He is one of the more than 50 Chinese workers who claim a New Zealand-based recruiter misled them about their pay and working rights.

Mao, along with 18 others, were given a limited visa to let them keep working while they helped Immigration New Zealand's investigation.

However, that visa doesn't qualify for the new fast track residency programme. Mao said that's unfair.

"We hope that Immigration New Zealand could treat us how they treat others," he said.

Another worker, Sheng Canhong, said men from the group who don't help the investigation, were not given these limited visas and it was ironic that some of them are now able to progress towards residency.

"They have transited into a work visa, some has a three-year work visa, but just the more than a dozen of us were singled out by Immigrations and were given this type of visa," he said.

"Why they can, but we can't? It seems to be a very simple question."

Advocate for Unite Union Mike Treen has been helping the men with their claims against their employers since early 2019. He said it makes no sense for this group of men to be treated differently.

"Some of those workers have become eligible for a pathway to residency, but the ones who are helping the government in this prosecution do not have a pathway to residency. We think that's just stupid and unfair," he said.

Treen said New Zealand needs them too.

"They're all workers who are needed in the construction industry in New Zealand, and the reasons that we gave pathways to people who are on regular visas still stands for these as well."

Anu Kaloti, president of Migrant Workers' Association, said these workers are helping preventing more people from falling victim of migrant worker exploitation.

"The limited purpose visa holders, they're actually doing the government a favoured by staying here and being witnesses to a very important investigation in the case of Chinese construction workers," she said.

"If we don't have those people stay on, we may never get to the bottom of this racket that these people were a victim of and then we will have more people impacted by it."

Kaloti said the workers have been here long enough and have been through lots of hardships too.

"They were badly hurt. They were grossly exploited. They were part of what I would say is like a human trafficking ring so you know from that perspective that the right thing to do here would be to include them in this new residence visa," she said.

In a statement, Andrew Craig, manager of immigration policy at the Ministry of Business, Innovation and Employment, confirmed that limited visa holders are not eligible for the new pathway because they are intended for short-term stays to allow people to achieve an express purpose.

"Other short-term visa holders are also excluded from this one-off residence pathway, such as visitors, working holiday makers, seasonal workers and short-term critical workers," he said.

"The 2021 Resident Visa focuses more on settled, skilled and scarce migrant workers, reflecting their critical part in New Zealand's economy."