OTTAWA -- The Senate's spending rules didn't need to spell out every detail of what was allowed to be expensed because much of it was "self-explanatory" and "intuitive" for senators, the upper chamber's new Speaker told court Monday.

Two independent auditors -- those from Deloitte who were tasked with reviewing Sen. Mike Duffy's expenses, and another group from KPMG who audited the Senate's finances -- decided that the rules were unclear and, to paraphrase KPMG, lacked enough detail that they could be misunderstood by senators.

Many specific circumstances aren't laid out in the rules, Sen. George Furey testified at the criminal trial of Duffy, who has pleaded not guilty to 31 charges related to his housing and travel expenses.

Furey said there wasn't anything in the rules to prevent him from hiring someone to mow his lawn on the Senate dime, "but I know intuitively that I can't do that." The same went for using Senate funds to raise money for a friend's run for another political office, he added.

Pressed by Duffy's lawyer, Donald Bayne, Furey said nothing exempts a senator from "their own personal responsibility to use probity and prudence in spending public funds." He said the overriding theme of the Senate rule book and the senators' handbook is that each senator is a trustee of the public purse and should keep that in mind when filing expense claims.

Furey's day-long testimony marked what was the final day of witnesses called by the Crown in Duffy's case.

Duffy has pleaded not guilty to 31 criminal charges stemming from his travel, office and housing claims.

Furey was on the three-member executive of the Senate committee that oversaw Duffy's spending audit in 2013 and the Senate's spending rules. Prime Minister Justin Trudeau appointed him Speaker of the Senate last week. Furey arrived in the courtroom Monday followed by lawyers from the Senate.

Duffy's defence team has long argued that the Senate's spending rules were unclear and ambiguous enough that Duffy could not have broken any rules, including those governing his duties as a senator, because the term was loosely defined.

Furey told the court that senators are given broad leeway when it comes to how they define parliamentary business given the broad range of activities they undertake.

A 2010 outside audit by firm Ernst &Young found "a lack of clear guidance and criteria" that would help senators understand what was a "parliamentary function" and what the Senate would pay for.

Furey told the court that the Senate's internal economy committee, which oversees spending, was sometimes told by administrators that senators had questions about what they could charge for and what constituted a "parliamentary function."

Furey said senators were urged to ask the Senate's administration or the committee that oversees spending about any rule if they had any question, concern or misunderstanding about the rules and to do so before they filed an expense claim.

Bayne also continued to make his case that the Senate didn't always follow its own rules. He pointed out that Duffy didn't always sign the monthly attendance sheets that senators file, which show how many sitting days they missed and for what reason.

Instead, someone from the Senate clerk's office would stamp it or sign it with the words "deemed as signed" -- a practice the clerk's office followed if a senator failed to sign off on the attendance sheets on time.

Furey also said he never signed a blank expense claim, something Duffy's former assistant has testified the former Conservative senator did to facilitate paperwork.

Furey said he spoke to his Senate Liberals in caucus meetings about the issue and advised them against it, adding that Conservative senators were given the same advice behind closed doors by senior Tory senators.

Furey called signing blank expense forms "poor practice," but said he had no first-hand knowledge of anyone doing it. When asked if he would do it, Furey was blunt: "I would not pre-sign a document because it would be open to abuse."

Bayne unsuccessfully argued that Furey shouldn't be allowed to testify because he was only there to provide opinions, not facts, about Senate spending rules.

Earlier in the day, Bayne told court that Furey told investigators looking into Duffy's spending that there was no need for rules defining primary and secondary residences for senators because the distinction was "self-explanatory."

According to Duffy's lawyer, Furey has already told investigators that a primary residence was where a senator's spouse and dog lived and where "your local pub is."