A one-page letter that the federal government has spent a year and a half and tens of thousands of dollars trying to keep from public sight raises explosive questions about Barnaby Joyce's conduct and "integrity".

The March 2015 letter written by Paul Grimes, the former head of the Agriculture Department, was sent 10 days before he was sacked amid fallout from the so-called "Hansard-gate" affair in which the transcript of some of Mr Joyce's statements in Parliament were quietly altered.

Mr Joyce has always blamed a rogue staffer acting without his knowledge and on March 16 last year - three days after Mr Grimes was terminated by the Coalition - he was asked on the ABC's PM program "does Paul Grimes agree with that version of events?".

"He's never told me if he has a different version of events. That is the truth, that is what happened," Mr Joyce responded.