The peak trade union body for NSW is taking the state government to the High Court, confirming it has filed a challenge against "outrageous" electoral funding laws.
The action will argue legislation passed by state parliament in May breaches the "constitutional right to political communication" by threatening to jail union members who engage in joint election campaigns.
The application is backed by the NSW Nurses and Midwives Association, Electrical Trades Union, NSW Teachers Federation, United Services Union and Health Services Union, according to a Unions NSW statement issued on Saturday.
"I understand politicians are sensitive to criticism. But that's no excuse to jail their opponents," Unions NSW secretary Mark Morey said.
"This is an outrageous abuse of power and that's why we are seeking an urgent High Court hearing."
He's concerned trade unionists could face serious consequences for activities, such as sharing the costs of airing election-related television advertising or publishing their logos on joint campaign material, considered "perfectly legal" at the last two elections.
Mr Morey hinted legal action would be taken in May when state parliament also capped electoral expenditure from third-party campaigners including unions, at $500,000 - less than half of the existing cap.
When the Electoral Funding bill was passed, NSW Premier Gladys Berejiklian said: "Our reforms will drive greater integrity, transparency and accountability in our electoral funding regime and help protect our democratic values and freedoms."
She said the reforms, applying to the next NSW election in March 2019, introduce important changes to reduce the risk of corruption and undue influence, and promote compliance.
The NSW government suffered a defeat in the High Court in 2013 when it last attempted to crack down on third-party donations.
Unions NSW led the constitutional challenge which resulted in the government being ordered to pay costs of around $600,000, according to the union.