Then-senator Raymond Lavigne was convicted in 2011 of fraud and breach of trust for using Senate funds for work on his property and for travel by staff members.
In 2006, a special Senate committee reviewed his expenses in private and decided to refer the matter to the RCMP. Lavigne and his lawyers participated in the hearings.
But the transcripts of those Senate proceedings were kept out of Lavigne's trial, as the Crown asserted parliamentary privilege. The idea behind parliamentary privilege in Westminster systems is to give lawmakers the freedom to do their work and manage their affairs with a certain immunity from criminal and civil law.
Now, the Senate's lawyers argue in a court affidavit that the details of a 2013 internal audit on the residency status of all senators should also be kept out of court.
They took the position in response to a request from Duffy, who wants to use the audit in his defence.
This latest privilege case is being fought under the authority of the Senate internal economy committee and Speaker Leo Housakos.
"The legislatures retain inherent privilege over those powers which are necessary for them to function as legislative bodies," lawyer Max Faille wrote in a letter to Duffy's lawyer, Donald Bayne.
The letter goes on to quote from Lavigne's unsuccessful 2010 court bid to have the charges against him stayed because of the withholding of the Senate records.
"The matter of discipline of a member of the Senate is a matter 'internal to the Senate,' to be resolved by the Senate's own procedure and I conclude would be subject to the Senate's own procedure and subject to parliamentary procedure," Ontario Superior Court Justice Robert Smith in the Lavigne case.
Duffy has pleaded not guilty to 31 charges of fraud, breach of trust and bribery in connection with his Senate travel, living and office expenses. The trial is set to resume on June 1, with Justice Charles Vaillancourt expected to deal with the privilege issue.