Unions NSW has declared it will take the state government to the High Court to challenge what it claims are "outrageous" electoral funding laws.
The peak body says the action will argue legislation passed in May breaches the "constitutional right to political communication" by threatening to jail union members who engage in joint election campaigns.
The NSW Nurses and Midwives Association, Electrical Trades Union, NSW Teachers Federation, United Services Union and Health Services Union have joined the application to the court.
Unions NSW secretary Mark Morey says 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, Premier Gladys Berejiklian said the reforms would "drive greater integrity, transparency and accountability".
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.