National is giving reassurances that new wide-ranging powers for immigration officers won't be abused.

The Immigration Amendment Bill, which squeaked through Parliament last night with just enough votes to become law, aims to stop exploitation of migrant workers,

But opposition MPs withdrew support over worries the new powers are too broad.

Immigration Minister Michael Woodhouse told Parliament there's limits to the search powers.

"An immigration officer needs to believe on reasonable grounds that the premises are owned, occupied, or used by the employer where it is believed work is being done. So it's not just an ordinary residential home."

"Workers may be hiding in part of the property. It doesn not make sense in these cases for the immigration officer to turn around and go to obtain a warrant because the workers will have vanished by the time the immigration officer returns."