Workmates was open to all workers regardless of union membership, and sought to organise action on the shop floor, controlled by the workers themselves.
About 30 percent of plumbers and pipe fitters belong to unions, according to unionstats.com, a website that estimates union membership based on federal data.
The court decided that the order of invalidity in respect of trade-union membership would only come into force three months from the date of judgement.
Right-to-work laws prohibit employers from entering into agreements that make union membership and the payment of union dues a condition of employment.
