Allowing migrant workers to switch between employers without having to change their visa could lead to employee-poaching, an immigration consultant says.
The new rules, which come into effect from 1 July, aim to attract an extra 5000 construction workers for the Christchurch rebuild, and will also see visas for low-skilled migrants extended from one to three years.
Immigration consultant Tuariki Delamere said the intention of the changes was good because it would cut down on the exploitation of workers.
But he said there could be unintended consequences.
"Unscrupulous employers, they'll let some employer do all of the work, all of the recruiting overseas, and then they'll go and poach them. That's going to happen, almost certainly.
"And you can't blame the migrant worker. He's going to be offered significantly more."
The new rules include an accreditation scheme for labour-hire companies, which are one of the biggest employers of migrant workers in Christchurch.
Mr Delamere said this was overdue but would require ongoing enforcement.
"You can put the rules in place but if they don't go around enforcing them, it's like, so what? So I guess it's watch this space, we'll see what happens, because there are certainly very dodgy labour-hire companies out there."
Immigration Minister Michael Woodhouse said the Labour Inspectorate would ensure labour-hire companies continued to play by the rules once they gained accreditation.
He was not worried about rival employers using the new rules to poach workers.
"While that risk is real, I don't think it's that high. We also have right now a number of employees who have applied for a variation of their visa conditions to enable them to work for other employers and 98 percent of those applications are approved."
Engineering, Printing and Manufacturing Union (EPMU) spokesperson Ron Angel said the changes would make a big difference.
He said migrant workers were vulnerable and too easily ripped off by unscrupulous employers.
"The employer says to you, if you give me any trouble, I'll call immigration and get your visa withdrawn and you'll be thrown out of the country.
"So everybody's hesitant to raise any issues. You know, and they've spent an awful lot of money getting here. They just want to come here and work."
Mr Angel said he had seen plenty of examples in Christchurch of workers not being given a fair deal.
"I was talking to another young man today whose agreement says the employer will endeavour to provide 50 hours per week. But he got rained off today and he got sent home without pay. For that guy, he's going, well, I've got a wife and three kids, if it rains for a week, I've got no pay for a week. Now, to my mind, that's exploitation."
Israel Cooper, the director of construction firm Build Tech, said the onus was on employers to do all they could to hang on to skilled staff.
"Organisations need to be competitive and on their game. So you know, that's just part of the rebuild and having a tight labour supply."
Mr Cooper said labour supply would remain tight with the rebuild now moving away from quake repairs and into larger commercial projects.