Boisvenu, who was Prime Minister Stephen Harper's point person on law and order issues in Quebec, is one of nine former or current senators who will be referred by the auditor general to the RCMP for criminal investigation.
CBC News has learned the nine includes one sitting Senate Liberal, five former or retired Liberals and two retired Conservatives.
Liberals Rod Zimmer (Man.), Rose-Marie Losier-Cool (N.B.) and Marie-Paule Charette-Poulin (Ont.) and Conservatives Donald Oliver (N.S.) and Gerry St. Germain (B.C.) were all found to have major problems with ineligible expenses. The sitting Liberal and two other former Liberals have not been identified.
Boisvenu announced his "voluntary" removal from the Conservative caucus in a statement late Thursday, adding he will explain his decision in more detail in the coming days.
Last year, Boisvenu was cleared of wrongdoing by the Senate ethics officer after attention was drawn to living expense claims filed when he was found to be dating his political assistant.
Zimmer made headlines as a sitting senator in 2012 over a disturbance on an airplane with his wife.
Another 21 senators, including the three most powerful figures in the Senate, will be flagged to repay inappropriate expenses. Those include Senate Speaker Leo Housakos, government leader Claude Carignan and Opposition Leader James Cowan.
Housakos and Cowan have vowed to take the auditor general's conclusions to an arbitrator appointed by Housakos to settle disputes over the auditor general's findings. Carignan said one of his staff members has repaid about $3,000 in expenses.
Housakos told CBC News he would act on the auditor general's recommendations.
"If there's recommendations in that report to refer specific cases to the RCMP — I've said it before, I'll say it again, and absolutely we will do it," he said. "There is no room in this institution for deliberate impropriety. None. I won't accept it, my colleagues won't accept it, the people of Canada won't accept it."
Report public Tuesday
The auditor general's report will not be made public until next Tuesday but copies of the report were handed over to the Senate and the Prime Minister's Office on Thursday.
It's been nearly two years since the auditor general was called in to review spending by senators, including travel and living expenses. The review included 116 current and former senators over several years.
The spending habits of suspended senators Mike Duffy, Patrick Brazeau and Pamela Wallin, as well as retired senator Mac Harb have already been investigated by the RCMP.
Duffy has pleaded not guilty to 31 charges of fraud, breach of trust and bribery in an ongoing trial in Ottawa. Trials for Brazeau and Harb have not begun, while Wallin has not been charged.