Centrelink is deliberately ripping-off thousands of Australians caught up in its data matching "robo-debt" program, with managers telling public servants at the agency to enforce debts they know are bogus, according to explosive new claims.

The serious allegations have emerged as a second Liberal backbench senator breaks ranks and criticises the data matching effort which the government hopes will add $4.5 billion to the budget bottom line.

Another whistleblower has come forward with an insider's account of how the "debts" are being pursued, alleging that glaring errors are deliberately ignored by Centrelink to allow it to extract money from its clients while others are being told they must repay "fictitious payments".

Centrelink denied all the allegations on Thursday saying the claims "do not accurately represent how the system works."

Pensioners and other struggling members of the community are being hounded for "recovery fees" unfairly added on top of their debts by Centrelink, according to the whistleblower's statement, published on Thursday by left leaning advocacy group Get-Up and Centrelink's main workplace union the CPSU.

The union says it has independently verified some, but not all, of the whistleblower's allegations while Get-Up pledged on Thursday to heap more pressure on the Coalition to rethink its strategy and will look to open up a legal front in its fight against the robo-debt campaign.

The insider, who has defied public service bosses' threats on leaking against the program, also alleges that Centrelink managers are well aware that bogus debts are being pursued and are ignoring pleas from compliance staff to take a fair approach to the debt recovery process.

"We are struggling with our consciences and pushing back against our leaders daily," the whistleblower wrote.

"We are telling the...helpdesk that what we are doing is wrong.

"I see these reviews every day and I am horrified at what I am being directed to do."

The insider alleges the rip-offs operate mainly by inflating a person's reportable income, counting money that should be outside reportable bounds, counting someone' income twice, making up Centrelink payment that were never paid, unfairly adding recovery fees to a debt or "corrupt reviews" where compliance officers are ordered not to correct an error.

But Centrelink media spokesman Hank Jongen denied the accusations in a statement posted online on Thursday, saying the claims about doubling income, non-assessable income, fictitious payments, false recovery fees and corrupted reviews were all incorrect.

"Some of our staff believe that intensive one-on-one management of recipients is always required," the spokesman said.

"As with other online initiatives in Centrelink, this intensive support is still available for those recipients who need it and complex cases.

"Many recipients prefer to manage through an online system, in their own time, rather than dealing with a staff member.

"Also, some staff do not welcome technology driven change."

Meantime, Liberal senator Jonathon Duniam has joined his senate colleague and fellow Tasmanian Eric Abetz in criticising the robo-debt push.

Senator Duniam has written to the Human Services minister, Alan Tudge, to express concerns about the system.

"The debt recovery process has clearly caused unnecessary angst for people," Senator Duniam said on Thursday.

"I've written to the minister expressing the concerns raised with me and seeking an urgent fix to this situation."