"Four or five years ago I thought this was a really rare occurrence," manager of Slater and Gordon's Comcare team Abraham Ghaleb said. "Whereas now it's an issue that I have to turn my mind to with every case that I deal with."

Figures released to a Senate committee show that Comcare has upped the rate at which it issues overpayment letters to recipients by 77% from 2014 to 2017. The letters, which are the result of formerly successful claims being revoked or changed, numbered 15 in 2014, rose to 37 in 2015, up to 71 in 2016 and 64 in 2017.

Comcare National Practice Group leader at Maurice Blackburn Georgia Plunkett-Scott has described the increase in reviews and overpayment letters as "alarming". Ms Plunkett-Scott said the increase in overpayment letters has led to some clients choosing not to pursue appeals in the Administrative Appeals Tribunal.

"They're so frightened that Comcare might do a review of their file, getting a new report, and that report saying something completely different to all the other reports on file, and all of a sudden overturning it, and requiring them to pay back or have a liability to Comcare."

"It is both financially and emotionally devastating for our clients when they receive an overpayment letter from Comcare," she said.

Mr Ghaleb has savaged the agency, accusing it of using a "bullying tactic" on people who are already in a vulnerable position.

"It's absolutely terrifying for people and it's an abhorrent behaviour for a Commonwealth department," he said.

Mr Ghaleb said the increasing rate at which overpayment letters are issued had led to changes in the way he manages cases where public servants are making claims for compensation.

"When I'm getting the evidence together [the prospect of an overpayment claim] always has to be in the forefront of my mind, 'is this a potential argument that Comcare will raise?' and 'how do I deal with it?"

Comcare says there has been no change in approach or policy that has led to the increase in overpayment letters and it is not clear if the increase in claims for people to return money has contributed to the insurance agency's recent healthy budget position. The rate at which debts to the agency are waived or written off is also on the increase.

Despite the lack of clarity about just how much money is being paid back, Mr Ghaleb believes issuing the letters is financially motivated.

"These figures are more than concerning. They're in my view targeting legitimate claims and perhaps even longstanding claims for the bottom line," he said.

Australian Federal Police employee Danielle Small won a case in the Administrative Appeals Tribunal last year after Comcare reviewed her case and found it was no longer liable to compensate her for injuries. Without the tribunal's decision, Ms Small could have been looking at a $550,000 debt to the agency.

Ms Small said she is relieved that the process is over, but that it took a heavy toll on her, both with legal fees and emotionally.

"I've gone into a very dark place and I've definitely considered those kind of dark thoughts," she said.

Ms Small said that since her case was covered by Fairfax Media she has been contacted by people in the same position.

"At least half a dozen people have got in contact with me that are going through very similar situations, some within my organisation, others outside, with very similar stories," she said.

Ms Small described the feeling that she was one of many people to go through a similar process as "disturbing".

While decisions made by Comcare can be appealed to the Administrative Appeals Tribunal, if an overpayment letter is issued following a decision by the tribunal, Comcare recipients are not able to appeal to have amount waived except within Comcare itself, according to Ms Plunkett-Scott.

"It's an access to justice issue," she said.

A Comcare spokesman refuted the suggestion that the increase in Comcare letters was financially motivated.

"Case reviews and debt recovery have had little impact on the Comcare scheme's financial position," the spokesman said.

"The scheme achieved full funding (100 per cent ratio of assets to liabilities) in 2016-17 for the first time in seven years. The biggest drivers of this recovery were sustained reductions in new claims, a strong focus on early intervention and better return to work outcomes."

The spokesman also denied the agency was using bullying tactics against claimants.

"Comcare strongly rejects any suggestion of improper behaviour," the spokesman said.