Just how hard did our federal politicians fight to keep their generous parliamentary perks? We may never know for sure.

Somewhere in the Department of Prime Minister and Cabinet is a document that sets out what more than 50 politicians told the entitlements review sparked by the Bronwyn Bishop choppergate scandal.

It's understood most of them argued strongly to keep their perks in place – and they were largely successful. But public servants are doing everything in their power to keep details of the interviews secret.

A Fairfax Media Freedom of Information request for the document was initially refused by the department on the grounds that its release would "substantially and unreasonably" divert the resources of the department.