Employers are responsible for those working on their sites, even if they have unknowingly employed someone working in the country illegally, Immigration New Zealand says.

The comments come after suggestions that dozens of illegal migrants are working on Christchurch's earthquake rebuild.

Fairfax reported yesterday that several national recruitment companies and local migrant agencies were fearful that a migrant scam had begun to take root in the city.

Immigration New Zealand said it was aware Christchurch's rebuild could attract illegal migrants and it would carry out random work-site inspections around Christchurch this year. "The onus is on [employers] to make sure workers working for them are legally entitled to be working in New Zealand," a spokeswoman said yesterday.

Immigration NZ advised employers to check the passports of potential employers to make sure they had an appropriate visa. Employers should also check the immigration status of workers online using Immigration NZ's VisaView service.

Employers could face legal action under the Immigration Act if they could not show Immigration NZ they had taken "reasonable" steps to ensure their employees were working in the country legally. The spokeswoman said it was impossible to speculate on the number of illegal workers in New Zealand. However, the number of possible overstayers had been estimated, she said.

As of May 2011, Immigration NZ estimated about 15,030 people were still in the country after their temporary visas had expired.

Christchurch immigration adviser Mike Bell said anyone aware of illegal activity had a legal obligation to notify authorities, while those employing illegal workers had "no excuse".

"You're always going to get an underbelly who are trying to drive down wages, trying to subvert the system.

"I think in the building industry there's always been an element of this . . . and there's definitely the potential for this [in Christchurch].

"We know of a couple of examples where people have tried to pay people under the table." Employers could not hire someone on a work visa without first checking if local workers could do the job - unless the work was on Immigration NZ's skill shortage list - and those on working visas had to be offered market rates, Bell said. "Immigration is supposed to plug the gaps, not create competition for local workers."

He advised responsible employers to protect themselves from possible legal action by utilising the VisaView service.