The committee which oversees the upper chamber's internal budgets asked the Deloitte accounting firm to review the claims of Conservative Mike Duffy, Liberal Mac Harb and Patrick Brazeau, who was ousted from the Tory caucus earlier this year over an unrelated criminal matter.
The accountants were to look at their claims about where they live and their related expenses.
"Barring unforeseen delays, the committee chair, Sen. Tkachuk, announced that the reports on residential expenses will be received by the full committee May 9, 2013," the Senate said in a statement released Thursday.
"The committee will consider possible recommendations and report to the Senate as quickly as possible."
The Constitution requires senators to reside in the places they are appointed to represent. They must also own at least $4,000 worth of property in that province or territory.
The Senate launched a review of its housing allowances following media reports that three longtime Ottawa-area residents in the upper chamber were improperly claiming housing allowances.
The allowances — currently worth $22,000 a year — are meant to compensate senators who keep a secondary residence in the national capital region.
Only senators whose "primary residence" is more than 100 kilometres from Ottawa are entitled to the housing subsidy.
One of the three senators has already repaid his housing allowance.
Duffy, a former broadcaster who now represents Prince Edward Island in the Senate, is a long-time Ottawa resident with a full-time home in the capital, making him ineligible for the subsidy. However, Duffy claimed a cottage in P.E.I. as his primary residence.
He also requested an expedited P.E.I. health card, but he was turned down by the provincial government. Provincial tax records show Duffy and his wife are identified as non-resident owners of their island cottage.
Late Friday, Duffy revealed he repaid the housing allowance.
"I have always said that I am a man of my word," he said in a statement. "In keeping with the commitment I made to Canadians, I can confirm that I repaid these expenses in March 2013."
The Senate confirmed Duffy repaid $90,172.24.
Brazeau holds a Quebec seat and Harb, a longtime former Ottawa MP, sits for Ontario.