The union movement in New South Wales is taking the state government to the High Court, challenging electoral donation laws which were changed last year.
Only individuals can donate to political parties in NSW, which is causing concern amongst unions and the Australian Labor Party about how they will fund election campaigns.
In what is seen as a test case, Unions NSW, which has 60 affiliates, is challenging the laws, arguing that donations are a form of political freedom of expression.
The case will be filed in the High Court on Monday.
A spokesman for Premier Barry O'Farrell said the state government is confident the laws will withstand a legal challenge.
However, Greens MP John Kaye is worried about the challenge.
"This High Court challenge could take NSW back to the bad old days of NSW where money ruled the roost," he said.
The ABC reports other states will be keenly watching the outcome of the case.
Queensland Premier Campbell Newman has previously flagged intentions to implement similar laws.
Constitutional law expert Professor George Williams, from the University of New South Wales, said if the NSW laws survive the challenge it will set a precedent which other states could follow.
"That precedent I'm sure is something that worries the union movement and the Labor Party, because this is not the sort of law they'd like to see rolled out around the country," he said.
"It's a major test case," he said.