The Engineering, Printing and Manufacturing Union will be defending the rights of striking workers in the Supreme Court tomorrow after Air New Zealand subsidiary Air Nelson won the right to challenge a Court of Appeal decision.

The key issue is the definition of the work of striking workers which has significant implications regarding the employer's ability to employ strike-breakers during an industrial dispute.

The Employment Relations Act 2000 places limits on employers employing other workers to do the work of striking or locked out workers.

The Supreme Court will be deciding what the test is for an employer to decide what is the work of a striking or locked-out worker and, therefore, whether or not the employer is able to compel other workers to strike-break and/or bring outside labour to do so.

The hearing starts at 10am at the Supreme Court.

The EPMU is New Zealand's largest private-sector union, representing 45,000 working New Zealanders across 11 industries.