YVONNE PASCOE: I'm concerned for his future. That's really my biggest concern now. What will happen when we are no longer here?

PAUL FARRELL, REPORTER: Christopher Pascoe loves helping his mother Yvonne out in the garden.

YVONNE PASCOE: He has epilepsy which is not totally controlled. He has poor fine motor coordination, poor eyesight but he is actually, he is a very hard worker and pretty sociable.

PAUL FARRELL: Eighteen months ago Christopher received a debt letter from Centrelink.

YVONNE PASCOE: It was a complete shock and surprise and thinking errors have been made or, this can't be true.

PAUL FARRELL: It accused Christopher of owing more than $15,000.

CHRISTOPHER PASCOE: When I found out, as you would imagine, I was real shocked. I was just stunned. Exactly why so much money?

PAUL FARRELL: Christopher received the disability pension while he was working part-time at a supermarket.

YVONNE PASCOE: The unfairness in the fact that Christopher knew nothing about it, the facts were just presented to us about 18 months ago as, you know, this is a complete deal.

PAUL FARRELL: Yvonne was determined to get to the bottom of the alleged debt.

YVONNE PASCOE: I set out with the intention of trying to find out what really has gone on and I was not able to find out, at any stage, where the figures that Centrelink had, who had given them those figures, where they'd come from. Centrelink have never been able to tell me.

(On telephone) Good afternoon. My name is Yvonne Pascoe.

PAUL FARRELL: She has made dozens of calls to Centrelink.

YVONNE PASCOE: (On telephone) Thank you, yes, I can wait.

Each time I spend anywhere between half an hour and an hour and a half talking to someone. I keep being passed from one person to another because I wouldn't be quiet and go away.

(On telephone) Can't blame everything on the computer system, I don't imagine.

That's the most frustrating part - that there is just no headway at all, no headway at all.

PAUL FARRELL: Her patience paid off when Centrelink admitted it had made a mistake.

YVONNE PASCOE: And there had been some review, internal review, then they discovered so-called errors in their organisation and it has been reduced to just a tad under $11,000.

PAUL FARRELL: Yvonne is now gearing up for a public fight against Centrelink in the Administrative Appeals Tribunal.

YVONNE PASCOE: It's really disability bullying to me because it's just gone on.

PROF. TERRY CARNEY, UNIVERSITY OF SYDNEY: When somebody with an intellectual incapacity is dealing with Centrelink, they're usually not in a position at all to understand when it is that they have to report things or precisely what it is that they have to tell Centrelink about.

PAUL FARRELL: Former tribunal member, Terry Carney, used to preside over hearings on robodebt cases like Christopher's.

TERRY CARNEY: I decided some cases like that and unlike the case I think that you're referring to, I took into account that Centrelink has procedures that allow someone else to be appointed, either to receive correspondence or to receive correspondence and actually manage the payment.

And when that's not done, then that is one of the possible special circumstances that can be a basis for waiving the debt.

PAUL FARRELL: 7.30 has spoken with dozens of welfare recipients who have had their debts reduced or waived entirely but only after they've challenged Centrelink.

Hundreds of people are now taking their fight to the Administrative Appeals Tribunal which reviews government decisions.

VIDA CARDEN-COYNE: It is difficult to stand up and say you want assistance. It's not something that you want to have happen.

PAUL FARRELL: Vida Carden-Coyne was grateful for receiving the parenting support payment from Centrelink in 2014.

VIDA CARDEN-COYNE: My partner and I had split up, we were both going through quite a lot financially.

We didn't expect to be in the situation that we were in and then to be told that you've done something wrong, and implied that it's deliberate, is really quite traumatic.

PAUL FARRELL: She, too, received a robodebt notice from Centrelink.

VIDA CARDEN-COYNE: I was frightened and I didn't understand where it had come from. I couldn't actually comprehend how I could have a $6,000 debt to Centrelink.

PAUL FARRELL: Centrelink's claims for alleged debt varied wildly.

VIDA CARDEN-COYNE: There's been three different amounts this year that I've been told I owe.

PAUL FARRELL: It eventually reduced her alleged debt to just over $1,300.

The Administrative Appeals Tribunal ruled she had to pay back this amount but she isn't prepared to accept their decision.

VIDA CARDEN-COYNE: How long are you going to be pursued for this? I mean, am I going to be pursued right up until I'm an old lady?

PAUL FARRELL: Are you going to continue fighting this debt?

VIDA CARDEN-COYNE: I will appeal it and I'm actually thinking about joining the class action.

PAUL FARRELL: In a statement, the Department of Human Services told 7.30 it was unable to comment on cases before the Administrative Appeals Tribunal and that it encouraged welfare recipients to contact Centrelink if they experienced issues.

TERRY CARNEY: An immoral raising of a debt against somebody that they don't actually owe to the government. It's a kind of a tax on people on social security.

PAUL FARRELL: Yvonne Pascoe was preparing to front the tribunal on behalf of her son, Christopher, when she received some welcome news.

After 7.30 contacted the department to raise questions, their lawyers contacted Yvonne with an offer to waive the rest of his debt.

YVONNE PASCOE: I've been dealing with this debt for Christopher as a nominee for 18 months.

Hundreds of hours of phone calls, lots of arguments, all this sort of thing going on and since the 7.30 Report, it is too coincidental for there not to be some connection.

PAUL FARRELL: She is still undecided whether to accept their offer.

YVONNE PASCOE: It is like a sort of magic wand or something.

It seems unreal and that's why I'm being, I suppose, a bit mistrustful. I want it explained to me in detail the why's and how's of how this has happened and who has decided to do this.

I just can't understand how it could have gone on so long and no-one's happened to notice.