OTTAWA -- A letter written last month by Conservative Sen. Mike Duffy suggests he was tipped off about irregularities in his expense claims by the chairman of the committee that was investigating them.

The letter suggests fellow Conservative Sen. David Tkachuk, head of the Senate's internal economy committee, told Duffy that an audit had found he collected more than $1,000 in living allowances while on vacation in Florida.

"Following our informal conversation, Tuesday evening, I went through my files for January 2012," Duffy says in the April 18 letter, obtained by The Canadian Press.

"I discovered that through a clerical error, per diems were inadvertently charged for several days when I was not in the National Capital Region."

The informal conversation referenced in the letter took place April 16 -- the same day Tkachuk was briefed by the auditors on their findings.

In the letter, Duffy blames an inexperienced, temporary staffer for the mistake. "This claim was clearly not appropriate and I will reimburse the Senate without hesitation."

In March, Duffy reimbursed the Senate $90,000 for a housing allowance he said he may have mistakenly claimed due to confusing paperwork.

Sources -- speaking on condition of anonymity because they weren't authorized to discuss the matter -- said the letter touched off an explosive meeting of the committee Thursday, with Liberal senators demanding that auditors go back to review Duffy's expenses further.

Conservative senators, who comprise a majority on the committee, rejected that idea.

The audit of Duffy's housing allowances -- as well as two other senators -- is to be tabled shortly in the Senate.

The audits deal with the housing allowances claimed by Sen. Patrick Brazeau, formerly a Conservative, and Liberal Mac Harb. A separate audit is still being conducted into the travel expenses of Conservative Sen. Pamela Wallin.

Sources say the audits indicate Brazeau received about $30,000 in housing allowances he was not eligible to claim and Harb about $50,000.

Duffy and Harb, who attended a portion of the committee's meetings Wednesday night and Thursday morning, played down the significance of the audits.

Harb -- who was accompanied by his lawyer, former Supreme Court justice Michel Bastarache -- said he expects to be vindicated by the audit into his claims.

A senator whose primary residence is more than 100 kilometres outside the national capital area is entitled to claim up to $22,000 a year to compensate for maintaining a secondary home in Ottawa.

Duffy, a former journalist who has lived in the capital for years, claimed his cottage in Prince Edward Island as his primary residence, although he appears to spend little time there.

Harb, who represented an Ottawa riding as an MP and owns several properties in the capital, claimed a bungalow in Westmeath, near Pembroke, Ont., as his primary residence. He put the bungalow up for sale several weeks ago.

Brazeau, who had been living with his girlfriend just across the river from the capital in Gatineau, Que., claimed his father's home in Maniwaki, Que., as his primary residence.

In an unrelated matter, Brazeau was forced to take a leave of absence from the Senate after he was charged with assault and sexual assault in February.

The furor over housing allowances erupted last fall and has cast a pall over the already much-maligned upper chamber. It has also raised questions about whether Duffy and Wallin are entitled to sit in the Senate at all.

A senator is required by the Constitution to reside in the province he or she was appointed to represent. Duffy was appointed by Prime Minister Stephen Harper to represent P.E.I., Wallin to represent Saskatchewan.

Wallin's travel expenses suggest she spends little time in her home province. But she maintains that's because there are few direct flights to Saskatchewan and only direct flights are counted by the Senate as travel to one's home province.