As scrutiny is applied to the entitlements of federal MPs following the over former Speaker Bronwyn Bishop's $5000 helicopter charter, details of the mystery NSW MP's failure to repay the money remains shrouded in secrecy.

The money was paid out under the so-called Sydney allowance, which is available to MPs who live at least 70 kilometres from NSW Parliament House or represent an "outer non-metropolitan electorate".

It is designed to cover overnight accommodation and other costs in Sydney while on parliamentary business.

Ministers, senior office holders and party leaders may claim up to $278 per day for 180 nights, or $50,040 a year.

Other office holders and ordinary MPs may claim the same daily rate, but for between 105 and 140 nights.

MPs may claim their total annual entitlement up front for a financial year, but must repay any unused portion by the following September 30.

However, in June an auditor-general's report highlighted that one MP had failed to pay back more than $25,500 in Sydney allowance claimed upfront as at March 31 - six months after the due date.

The same report said seven MPs had failed to return their unspent Sydney allowance by September 30, 2014.

"While six members subsequently repaid their unspent money after the deadline, one member still owed over $25,500 at 31 March 2015," it said.

Executive manager of the Department of Parliamentary Services, Robert Stefanic, said the auditor-general's report showed members "are generally complying with the rules in relation to entitlements."

"All members mentioned in the Auditor-General's report had either repaid or were in the process of repaying these allowances as at 31 March 2015, in accordance with the relevant rules and legislation," he said.

Asked whether the MP had yet repaid the $25,500 owing, Mr Stefanic said: "Consistent with long-standing practice, the Parliament is unable to comment on matters relating to individual members."

Do you know more? srnicholls@smh.com.au