A recruitment company criticised for an illegal clause added to its contracts barring migrant workers from joining a union is blaming an authority in the Philippines for the mistake.

Allied Work Force (AWF) is under investigation by Immigration New Zealand and workplace inspectors.

Its chief executive Simon Bennett said in November a manager was asked by a Filipino agency, the Philippines Overseas Employment Authority, to send a form on the company letterhead, which included the errant clause.

"He did that unwittingly - he felt it was more about the obligations on us as an employer, because it was dealing with what happens if someone needed to go home, the commitment on our part to pay for their airfare et cetera.

"Certainly it was not a legal addendum to our individual employment agreements - which had been signed some months previously - as it is plainly contrary to New Zealand law."

The Filipino workers had received comprehensive information about living and working in New Zealand, including their right to join a union.

"We are confident that our individual employment agreements with the 99 Filipino workers are legally valid and binding and that we can continue to provide an invaluable service to Auckland contracting companies in need of skilled workers."

AWF accepted "the mistake should not have happened", he said.

The manager who sent the letter had resigned due to the pressure of being in the media spotlight.

"He felt he had let us down, he was upset that he had a made a mistake that made a lot of noise and in the end had significant ramifications."

Immigration New Zealand manager Peter Elms said the department was working with AWF to establish the facts before deciding on what, "if any", action would be taken.