Unions in New South Wales are preparing a High Court challenge to state laws governing political donations.
AAP reports reforms, introduced by the state government earlier in the year, limit donations to individuals and ban contributions from unions, corporations, peak industry groups and community groups.
Campaign spending by politically affiliated organisations like unions would be included in a political party's expenditure cap.
"It effectively means there's no incentive or capacity for a union to take part in electioneering," constitutional lawyer George Williams told a forum in Sydney on Thursday.
Unions could lose their voice as they are no longer able to financially contribute to the ALP, the party established to promote their political objectives, Mr Williams said.
Unions NSW secretary Mark Lennon said the union would challenge the reforms to ensure its voice would be heard in the political arena.
"Freedom of speech... is a fundamental foundation stone of democracy in this country," he said.