The Court of Appeal says staff cannot be forced to do the work of striking colleagues.
The Engineering, Printing and Manufacturing Union had appealed against two decisions of the Employment Court that said employers could force staff to do the work.
In 2005, Atlas Specialty Metals ordered two employees who were not on strike to carry out the work two striking employees would have been doing. They refused, and were suspended without pay.
In 2007, Air Nelson employed contractors and Air New Zealand employees to carry out work which would have otherwise been done by striking employees.
The Employment Court ruled the employers had acted lawfully but the Court of Appeal says making workers do the jobs of striking colleagues is not what was intended by the Employment Relations Act.
The Court of Appeal says its decision should be easier for employers, employees and unions to apply during a strike.