By Dominic Giannini in Canberra

Australia’s peak union body has backed a spending cap at federal elections, arguing big business and billionaires couldn’t unduly influence votes.

Candidates will be limited to spending $800,000 in an election campaign and will only be able to receive donations of up to $20,000 from an individual under proposed reforms set to be introduced in parliament on Monday.

There will also be a cap on federal spending for non-political parties of $11 million, which covers unions and special interest groups like Climate 200, while registered political parties will have a ceiling of $90 million.

The Australian Council of Trade Unions has thrown its support behind the changes.

“We support these electoral reforms because it would mean that big business billionaires don’t get to buy votes in Australian elections,” ACTU assistant secretary, Liam O’Brien told AAP.

Fairness and transparency “have never been more critical”, he said, attacking Opposition Leader Peter Dutton’s threat to roll back industrial relation changes under Labor as a threat to worker’s rights like collective bargaining.

The reforms were, in large part, a response to billionaire Clive Palmer spending more than $120 million at the 2022 poll and tens of millions of dollars in 2019 to stop Labor’s Bill Shorten from being elected.

Mr Palmer has flagged a High Court challenge, saying they suppress freedom of speech and were a deal stitched between the major parties to limit political competition.

Special Minister of State Don Farrell argued the opposite, saying the caps levelled the playing field against billionaires seeking to influence elections with resources unavailable to the average voter.

The caps were designed in a way to best withstand a court challenge on the basis of freedom of speech, Senator Farrell said.

Independents who have called for more transparency and electoral reform are supportive of limiting the influence of big money and more donation disclosure changes.

They argue the reforms tilt the scales towards incumbents and major parties, with the scale of money from federal spending caps and an increase in the amount of money they receive per vote not available to independents.

The laws won’t come into effect until mid-2026, which is after the next federal election, due by May 2025.