An investigation into 10 unlawful Chinese workers should also probe whether agents and corrupt officials helped them secure visas, according to an immigration lawyer.
The men, who were working in construction, are in the process of being deported amid concerns they could be the victims of exploitation or human trafficking.
Immigration Minister Kris Faafoi has previously said it was an operational matter.
"We have been assured that the appropriate processes have been followed and Immigration NZ has not found any evidence of trafficking," Faafoi said.
"Agencies are satisfied further investigations around employment and immigration breaches can be carried out without the need for the men to remain in New Zealand."
Lawyer Matt Robson described that as "preposterous", as witnesses would be the basis of any prosecution brought against companies or individuals.
None of the men had undergone in-depth interviews with the Labour Inspectorate or Immigration New Zealand, he said.
In fact, the Labour inspector had asked their lawyer and the union for information, but Robson said they had neither the resources nor the prison access to interview 10 men with interpreters.
Robson suggested the two agencies might be working at cross-purposes and said the investigation needed to look at the workers' agents, here and in China.
"When they came to the airport in New Zealand, they had someone to meet them, they had people in China who prepared everything," he said. "All they did was pay the money, and sign whatever they were asked to sign and provide whatever documents they were asked for.
"But they have no knowledge of the process - they needed these agents even in Auckland to survive, they needed people to get them accommodation, transport to their various jobs."
One of the deportees, who escaped from a police car en route to the airport, wanted to get the wages that he was owed and collect his possessions, Robson said.
He had unbuckled his seatbelt at traffic lights, hidden and then followed landmarks to Dominion Road, before handing himself into police.
The visa process raised questions of possible corruption, Robson said.
"The workers that I've seen - the majority seem to have got their visas within about two days of application, that's what it says on the immigration sheets, and one of them was six days, but all of them are just lightning speed for Immigration New Zealand.
"I've been a lawyer in the immigration field for many decades, I've never got a visa, an ordinary visa, in that time, plus some of them got the ability to come in and out for five years, which is another unheard of for a person to come here.
"So, that's a major question, the chain from China to New Zealand, and the agents here. The minister has talked about a full investigation - well, you can't have a full investigation if you throw the witnesses out of the country."
Faafoi and Workplace Relations and Safety Minister Michael Wood have been approached for comment.