The federal government's workplace watchdog is offering amnesty to temporary foreign workers who assist in workplace exploitation investigations despite having no clear power to do so, potentially exposing them to the risk of deportation.

For more than a year the Fair Work Ombudsman has been publicly claiming that temporary migrant workers who breach their visa work rights will not be deported if they are also victims of exploitation and go on to help an investigation.

But Freedom of Information documents obtained by Unions NSW show the Ombudsman only has an informal arrangement with the Department of Home Affairs - formerly the Department of Immigration and Border Protection - rather than a binding agreement.

Fair Work Ombudsman Natalie James Credit:James Brickwood

In a document entitled "New FWO/DIBP referral protocols" it is made clear that for temporary visa holders who have a work entitlement Home Affairs "will generally not cancel their visa, detain or remove those individuals from Australia".