OTTAWA -- Just one day after Sen. Mike Duffy's sensational expense-claim trial ended in acquittal, the Senate is staring down the possibility of heading to court over more expense claims.

Today marks a Senate-imposed deadline for seven of its former members to hand over almost $528,000 in claims the federal auditor general ruled should never have been charged to taxpayers.

The seven were among 30 senators flagged in the June 2015 report; none of the seven chose to challenge the decision through an independent arbitration process.

The last of the senators who owed money after the arbitration process headed by former Supreme Court justice Ian Binnie repaid the Senate at the start of the week.

The Senate had vowed to go to court to recoup the cash from anyone who doesn't pay up on time; as of Friday morning, there was no indication the seven were about to pay up.

Conservative Sen. Leo Housakos, chairman of the internal economy committee that oversees Senate spending, said in a statement that the Senate is looking for an outside lawyer to help them with the impending civil litigation.

The seven who owe money are retired Liberal senators Sharon Carstairs, Marie-P. Charette-Poulin, Rose-Marie Losier-Cool, Bill Rompkey, and Rod Zimmer, as well as former Conservatives Don Oliver and Gerry St. Germain.

The expenses in question are not all that different from the ones that landed Duffy in court.

Duffy's lawyer, Donald Bayne, said his client was singled out for criminal prosecution even though the auditor general and Binnie ruled these 30 senators filed expenses that were not-Senate related. He said parliamentarians need to give more scrutiny to the expenses they charge to the public purse.

"I'm sure all of you, like me, could make a good argument that some of the things that senators are allowed to travel across the country for at considerable expense ... may or may not pass a value-for-money test," Bayne said outside the courthouse Thursday.

"There has to be that kind of assessment on Parliament Hill of expenditure of public money. Those kind of rules did not exist in Mike Duffy's day."

Duffy was cleared of 31 criminal charges by Ontario Court Judge Charles Vaillancourt on Thursday, ending a three-year saga with his expenses and allowing him back into the Senate with all the rights and privileges of his colleagues.

Vaillancourt ruled that Duffy's expenses didn't enter the realm of criminality even though they may raise eyebrows, including a consulting contract with his one-time personal trainer. The Senate's administrative rules allowed such spending, the judge said.

Those administrative rules are set to be updated as part of an overhaul of Senate spending guidelines and oversight stemming from the auditor general report.