When a Melbourne law firm announced a class action into the Coalition’s controversial robodebt scheme on Tuesday, Francine Wheeler didn’t waste any time.

A few hours after Labor’s government services spokesman, Bill Shorten, revealed Gordon Legal’s federal court challenge at Parliament House, Wheeler, 28, had already registered her interest in taking part.

“I just think it’s so important,” she says. “I feel that there’s no proper process that Centrelink are using to take this money off people.”

Since the Coalition expanded the robodebt scheme in 2016, more than 800,000 debt letters have been to sent current and former welfare recipients.

While the government has responded to sustained criticism by introducing more human involvement to what is essentially a data-matching program, it has been dogged by figures showing that about one in five debts issued have been wiped, waived or reduced. Last month Guardian Australia revealed that confidential documents showed the scheme would need to be expanded further to meet its budget targets.

For Wheeler, who had $1,785 garnisheed from her tax return in July over an alleged Youth Allowance debt from 2013-14, robodebt has been an unwelcome burden.

“It’s just been such a horrible weight on my shoulders,” she says. “It’s been so difficult.

Bill Shorten with Gordon Legal senior partner Peter Gordon announcing the launch of a class action against the Centrelink robodebt scheme. Photograph: Mike Bowers/The Guardian

“Basically what happened was they did the review and in the time of them doing it, they just automatically garnished my tax return. Before they’d even sent me a letter confirming that.”

Wheeler was among the more than 170,000 people who were contacted by a debt collector about their alleged overpayment.

“That really freaked me out because it’s a lot of money,” she says. “Also the number on the phone they give you, it calls Centrelink but it asks you to press a number for whatever debt collector has been engaged, and it just goes back to the debt collector.

“I didn’t want to talk to the debt collector. I wanted to talk to Centrelink.”

The Brisbane woman says she was forced to collect as many payslips as she could, but that proved difficult. The best she could do was gather old bank statements and submit them to Centrelink.

In February the agency said she owed them $4,500. After a review on the phone, it was $150. Later it was around $1,500.

“I don’t think that I owe them any money because I was honest at the time of reporting my income,” Wheeler says.

Gordon Legal was inundated with inquiries about the class action after it was announced at 1pm on Tuesday this week. The firm asked people to register their interest before it selects a test case and about 1,200 people had made contact in the first 24 hours.

“We think there is a strong legal basis and a strong factual basis [to proceed],” the firm’s senior partner, Peter Gordon, said this week.

The class action came about after Shorten contacted the law firm. Labor was critical of the program in opposition, particularly Shorten who made reference to it during the campaign. But Labor declined to promise to scrap the program.

Shortly after the new parliament convened, Shorten, now the opposition’s spokesman for government services, took some by surprise when he called for the government to scrap the scheme.

“Robodebt is very likely illegal,” Shorten said this week. “The scheme - including its reverse onus of proof – is at best legally dubious and should rightly have its legality determined by a court. It is right that this action be pursued by the lawyer who pursued justice in David and Goliath battles with Big Tobacco and Big Pharma.”

The class action follows a separate federal court challenge by Victoria Legal Aid, which also argues that the use of Australian Taxation Office data-matching to substantiate whether a person was overpaid their income support is unlawful.

The way it works is that Centrelink will average a person’s yearly income over the welfare recipient’s fortnightly income reporting periods to calculate whether a debt is owed.

This methods have long been criticised by welfare and legal groups, as well as grassroots campaigners such as the NotMyDebt group, but the government argues it is the responsibility of the welfare recipient to clear up any potential discrepancy by providing payslips and other evidence to allow for a more accurate calculation.

Critics say the process wrongly places the onus on people to prove they do not owe a debt, which can date back as much as seven years. The NotMyDebt campaign, which helps people take on the system, says the spectre of a class action is not an “instant fix”.

Still, many people caught up in the robodebt scheme expressed delight at news of the class action. Some said it gave them a sense of hope.

A Canberra woman, Jules Bray, 50, says she is considering taking part in the class action.

In 2017, Bray was forced to battle Centrelink over a $1,700 debt. “I was not allowed to see anyone face to face to talk about this,” she says.

“I tried to go into Centrelink offices. ‘Oh, we don’t have anyone qualified to talk about that. You have to contact the department, you can only talk to them over the phone.’

“And it was the most maddening experience trying to explain … They absolutely flat-out refused to see my rationale.”

Eventually, the figure was reduced to a few hundred dollars and automatically taken out of Bray’s income support payments. Critics say more vulnerable welfare recipients end up paying debts they may not owe without challenge.

The government services minister, Stuart Robert, has accused Shorten of a “stunt” and said Labor wanted to give hundreds of thousands of Australians “a leave pass” without saying where the lost revenue would come from.

He said data-matching had begun under Labor while Bill Shorten and Tanya Plibersek were the responsible ministers.

The former Labor government did introduce the process, but it was carried out on a much smaller scale, with significantly more human involvement.

The Coalition turbocharged the scheme in 2016, increasing the number of “compliance interventions” from 20,000 a year to 20,000 a week.

Despite her tax return taken, Wheeler believes she is one of the lucky ones. “It’s important for people who are in less fortunate position than me,” she says.

“I’m a native Australian speaker, I have a well-paying permanent job, and it’s been difficult for me. I feel terrible for anyone who may have barriers.

“People who don’t have access to the same level of communication, the internet or email, I just have no idea how they would fight a debt from Centrelink.

“I actually think it’s a human rights issue.”