The NDP says it's being subjected to an "unprecedented attack" by the majority Conservatives after being ordered to repay $1.17 million for improper mailings.

NDP House Leader Peter Julian called the issue of improper mailings a "witch hunt" and criticized the Conservatives for not providing information on their own mailings.

"This is an unprecedented attack on an opposition party where the Conservatives are refusing to actually provide any sort of information about their own mailings and aren't even giving information about what rules have actually been broken," Julian told CTV's Power Play on Thursday.

The party is calling on the Auditor General to review the mailing programs of all parties.

The NDP has also said it plans to go to court to challenge the verdict of the Board of Internal Economy, which found New Democrat MPs guilty on Wednesday of improperly spending $1.17 million in parliamentary resources on partisan mass mailings.

The multi-party committee found that 23 New Democrat MPs improperly used parliamentary resources to send 1.8 million partisan mailings to households in 26 ridings, including four that were about to have byelections.

The committee, which oversees the finances of the House of Commons and sets rules for MPs' spending, ordered the NDP MPs to reimburse $36,000 in direct mailing costs to the Commons. It said it did not have the power to compel the MPs to reimburse Canada Post for misuse of $1.13 million in free mailing privileges, known as franking, but said they have an obligation to do so.

Meanwhile, the minister responsible for Canada Post said she will ensure the NDP pay back every cent of free parliamentary postal privileges used to mail out nearly 2 million mailings.

Minister of Transport Lisa Raitt, who is responsible for Canada Post, said the Parliamentary rules regarding partisan mailings are very clear.

"It is not acceptable to use House of Commons resources to fund party offices or to send out partisan mail-outs. They know this. And as a result I expect they're going to pay back Canada Post," Raitt said during question period on Thursday. "But I also expect that they'll refuse to pay back Canada Post."

Raitt said she had spoken to the CEO of Canada Post Deepak Chopra "to make sure he understands what's happening."

"He does indeed take it very seriously," she added.

In a statement Thursday from her office, Raitt said: "Rest assured, every single penny that was misspent by the NDP will be paid back to hard working Canadian taxpayers."

Julian criticized Raitt for reaching out to Canada Post regarding the NDP franked mail when she refused to step in to prevent cuts to home delivery service, claiming that Canada Post is an independent, arm's length Crown corporation.

"So there's nothing we can do for the five million Canadians who are going to end up losing postal service, but she intervenes with Canada Post on a Conservative witch hunt," he said.

The NDP has decried a process that did not include the presentation of evidence, witness testimony or an opportunity for the party to defend itself.

"We actually have rule of law in this country which means a bunch of Conservatives in a secretive internal committee can't just make decisions without due process and following laws," Julian said.

Throughout question period on Thursday, Prime Minister Stephen Harper used almost every query from New Democrat Leader Thomas Mulcair to segue to the mailing controversy.

"The real question that Canadians are asking is when the NDP is going to return the public money that it took completely illicitly to use on partisan mailings and partisan offices under the guise of parliamentary spending," Harper said.

"The NDP knows, from top to bottom, that this was inappropriate, incorrect and fraudulent, and should do the right thing and return the money."

With a report from CTV’s Ottawa Bureau Chief Robert Fife and files from The Canadian Press