Mulcair made the comments Monday as he highlighted his so-called "Roll up the Red Carpet" tour, part of a national campaign to publicize his party's support for Senate abolition.
The campaign opened in May, with the first stop of the tour three weeks ago in P.E.I., home province of embattled Sen. Mike Duffy.
Both Duffy and colleague Pamela Wallin claimed expenses related to Senate business while they were actually attending party events.
"They were participating in these highly partisan activities with public money," Mulcair said outside the Senate chamber.
"So at the very least, the Conservative party should reimburse for the audit because it was an audit that was made necessary because the prime minister didn't tell the truth in the Commons in February when he said there was no problem with Pamela Wallin's expenses."
Mulcair vowed to keep the Senate on the political front-burner, despite the fact Parliament won't return until October. Prime Minister Stephen Harper has signalled his intent to prorogue Parliament and open a new session with a speech from the throne.
"In the last five weeks of Parliament, the prime minister showed up to answer questions exactly five times," Mulcair told an enthusiastic gathering of NDP staffers. "By the time the House finally reconvenes it will have been five months since he's had to answer a question.
"Well, Mr. Harper needs to answer for himself and he needs to answer to Canadians. If he thought the questions last spring were tough, he hasn't seen anything yet. We're just getting started."
Senate reform is likely to figure in the throne speech, which will outline the government's legislative agenda for the new session.
The Conservatives are awaiting a response from the Supreme Court of Canada on a variety of Senate reform questions, including how the upper chamber might be abolished.
Later Monday, Mulcair brought his anti-Senate campaign to the Nova Scotia legislature in Halifax, where he toured the building's Red Chamber, used by the provincial senate until it was abolished in the 1920s.
"We're proposing to do the exact same thing in Ottawa," Mulcair said as he posed for pictures with NDP Premier Darrell Dexter.
"You're dealing with party favourites, insiders, bagmen, fundraisers — people who owe a lot to the Prime Minister's Office. ... They're not there to take care of the provinces, they're not there to take care of Canadians. They're there for themselves and the party that named them."
Commenting on the resignation Monday of former Liberal Sen. Mac Harb, Mulcair said: "There's hope. One down, 97 to go."
The upper chamber has 105 seats, though there now are six vacancies after Harb's departure, with 99 members sitting — a corrected number the NDP issued in a later release.
Dexter repeated the call for an end to the Senate.
"We abolished the Senate here in 1928 and I've been through five or six general elections now, and nobody's ever asked me to bring it back.
"I don't think we miss it all that much."
— With files from Michael MacDonald in Halifax