The EPMU defended the rights of striking workers in the Supreme Court on Tuesday after Air New Zealand subsidiary Air Nelson won the right to challenge a Court of Appeal decision.

The Supreme Court has reserved its judgment on the issue, which revolves around the definition of the work of striking workers during industrial action and the right of employers to undermine workers' industrial action by using strike-breaking labour.

The issue arose after Air Nelson used strike-breaking labour to undermine a strike by EPMU members employed there in 2007. The EPMU took Air Nelson to the Employment Court, which ruled in Air Nelson's favour. The EPMU appealed that case to the Court of Appeal and had the ruling overturned, and Air Nelson subsequently took the case to the Supreme Court.

The outcome of the case will have major implications for union members as it will determine the ability of employers to undermine industrial action with strike-breaking labour.

A ruling on the case could take several months.