The Maritime Union says New Zealand signing an international maritime agreement will protect seafarers' rights only if foreign vessels are better monitored.
Dubbed the "seafarers bill of rights", the Maritime Labour Convention means shipowners can be convicted for mistreating their crews, if the abuse took place in New Zealand waters.
Previously, charges had to be laid in the country where the ship was registered.
The agreement followed several high-profile cases of abuse of foreign fishing crews on New Zealand-chartered foreign fishing vessels.
Secretary of the Maritime Union Joe Fleetwood welcomed the move but said more inspectors were needed to ensure boats complied.
"To make it a tiger with teeth we need to have more inspectors that can get down there and belt the ship owners into place."
"We had to take this step so that we can be compliant with the rest of the world."
He said foreign fishing vessels should be taxed to pay for the inspectors.