The former chair of the Senate's internal economy committee says a senator who is launching a legal challenge of an order to repay thousands of dollars in expenses "was given due process."

Senator Mac Harb, who quit the Liberal caucus in May after a Senate report concluded he must pay back $51,000 in living and mileage expense claims, was told this week he should pay back a total of $231,000 or face an extensive audit of his expense claims going back to 2005.

CBC News learned Friday that Harb is launching a legal challenge against the Senate in Ontario Superior Court.

In an interview airing Saturday on CBC Radio's The House, Conservative Senator David Tkachuk said the Senate's review of Harb's residency and expense claims was fair.

"Everybody knows where their primary residence is. He was given due process," Tkachuk told host Evan Solomon.

Harb's lawyers are expected to argue he was not given due process or a fair hearing by the Senate.

According to Tkachuk, Harb's peers "decided that his primary residence was in the city of Ottawa and therefore he owed them expense money for ... as many years back as we have records for."

"That was a decision of all the members from both parties."

At a May 9 meeting, the 15-member Senate committee that monitors senators' expenses reported it does not believe Harb's Pembroke, Ont. home is actually his primary residence.

Senators are allowed to charge living and travel expenses if their primary residence is at least 100 kilometres from Ottawa.

Tkachuk resigned as chair of the Senate's committee on internal economy this week for health reasons, after several months of controversy over audits into the expense claims of Harb and fellow senators Mike Duffy, Patrick Brazeau and Pamela Wallin. 

The committee tabled reports on Duffy, Harb and Brazeau on May 9 and expects to receive an audit of Wallin's travel expenses in July.

Brazeau has vowed a court fight over an order that he repay $48,000. Both Duffy and Brazeau are now sitting in the Senate as Independents.

Duffy report not a 'whitewash'

Tkachuk also told Solomon the committee likely would not have closed its review of Duffy's expenses had it known Duffy received $90,000 from the prime minister's former chief of staff, Nigel Wright.

The RCMP announced this week it has launched a criminal probe into the matter.

"At the time, we just received a cheque from Mike Duffy, which we received in March. It was cashed and paid for, and so his expenses were paid for. I think we came to the right conclusion on May 9. Who knew what was going to happen after the fact," Tkachuk said.

"As far as we were concerned at the time, the case was closed. As far as I'm concerned, what [the RCMP is] investigating now is a different matter," he said.

Tkachuk bristled at allegations that last-minute changes to the Duffy report amounted to a "whitewash," calling the suggestion "propaganda" by the Liberal party.

"We didn't whitewash the report. We changed the report. Every committee does that, they do it all the time, and everybody knows that. And it's a very rare time that the first draft of a committee decision is adopted by the committee," Tkachuk said.

Asked if the Senate's system of checks and balances failed in the case of Wallin, Tkachuk said "it was the system that did detect what she was doing."

"It was the system that reported it to internal economy. And it was the system who received money from her and it was the system that referred it to the auditors," Tkachuk told Solomon.

Wallin, who was forced to resign from the Conservative caucus during an audit of her travel expense claims, said in an interview with CBC's Peter Mansbridge she was sorry for the mistakes she made on her travel claims. She said she has repaid $38,000 so far.

Asked whether he thought the senators should be allowed back into caucus if they repay their ineligible expenses, Tkachuk said if he was the leader, "no, I would not let them back in."

Tkachuk starts preventive treatment for bladder cancer on June 18.

"It's kind of a unique treatment and I'm hoping it's successful," Tkachuk said.