There has been a call for an overhaul of the primary mechanism of accountability in the parliamentary entitlements system, after the ABC revealed two sitting parliamentarians ignored requests to personally sign off on their expenses over a period of years.

Key points: Calls for greater accountability in parliamentary entitlements

Calls for greater accountability in parliamentary entitlements Follows ABC's report on Freedom of Information documents obtained

Follows ABC's report on Freedom of Information documents obtained Documents showed two parliamentarians ignored requests to sign off their expenses

Since 2011, parliamentarians have been required to sign a form every six months certifying their expense claims are legitimate.

Freedom of Information documents obtained by Lateline showed that Labor frontbencher Stephen Conroy ignored 23 requests from bureaucrats to sign the certification forms over a period of four years.

The ABC understands he signed them in the past fortnight after learning of the FOI request, certifying around $1.8 million in expenses claimed since January 2011.

Independent Queensland MP Bob Katter has still not signed a certification form for any of his claims from 2011.

"It is not possible and would be very improper for him to sign a statement which sets out every single expenditure across his two offices when he is rarely in the offices," a spokeswoman for Mr Katter said.

She added that it would be "a great cost-saving to Mr Katter personally if he sat in the office in Charters Towers permanently".

More than 100 other MPs have failed to sign their half-yearly certification at least once.

The rules do not apply penalties for any parliamentarian who fails to sign the form.

But there are now calls for signatures to be made mandatory as part of the next overhaul of the system, announced following former speaker Bronwyn Bishop's so-called choppergate affair.

Professor Allan Fels, who was on the panel of the last root-and-branch review of the system that recommended the certification system, is one calling for penalties to be introduced.

"I believe that if they don't certify, that there should be a penalty of about 25 per cent, somewhat comparable to the penalty if there is an over-claim that has to be refunded," he said.

Independent South Australian senator Nick Xenophon called for changes that would "require MPs to pay back the money that's been spent on their entitlements unless they sign [the certification form] within a certain period of time".