The New Democratic Party of Canada has paid back a total of $344,468 it obtained in sponsorship revenue since 2006 after Elections Canada found the federal party guilty of violating political financing laws following a complaint filed by the Conservative Party of Canada.

In an interview with CBC News, Nathan Rotman, national director for the NDP, said the reimbursement will be reflected in the party's quarterly financial return, which is being submitted to Elections Canada today.

Rotman said the party returned the funds in April and May directly to the unions and organizations that purchased the advertising, with copies of the cheques sent to Elections Canada

In an interview on CBC News Network's Power & Politics on Monday, Poilievre accused the NDP of "covering-up" a scheme with labour union brass.

Poilievre told host Evan Solomon: "This was eight unions and seven other organizations who at three separate conventions, over five years, gave almost $350,000 of illegal donations to the NDP."

The Conservative MP said the money was "syphoned off" the paycheques of union workers and "illegally funnelled into the coffers of the NDP."

NDP MP Libby Davies refuted Poilievre's allegations calling them "venomous attacks against the NDP" and pointed to a case where the Conservatives had to repay $230,198 in an election finance dispute that became known as the in-and-out scheme.

"The NDP immediately co-operated with Elections Canada, went into compliance with the law and the money was returned. It was an honest mistake, a misinterpretation of the rules," said Davies.

The Conservative Party of Canada sent Elections Canada several photos of signage at the NDP convention in June 2011 that display logos of Canada's largest unions. (Conservative Party of Canada)

The election watchdog said the New Democrats broke the rules after accepting sponsorship money from large unions during the NDP's national convention last June.

Elections Canada noted that the sponsorships constituted contributions, and because parties can't accept contributions from unions, the money was considered ineligible.

Labour unions and other organizations purchased $40,860 in advertising during the party's 2006 convention in Quebec City, $102,500 during the 2009 convention in Halifax, and $102,500 during the 2011 convention in Vancouver for a total of $344,468 – GST included.

Until now, both the party and NDP Leader Tom Mulcair had refused to say how much the party had been forced to pay back after Elections Canada found it had violated political financing laws.

Rotman said the party believed it was complying with the law until Elections Canada said otherwise.

NDP leadership convention

In a statement posted on its website on Sunday, the Conservative Party vowed it would continue to "defend political party finance laws."

In June, Conservative MP Scott Armstrong also wrote to the Commissioner of Canada Elections asking the watchdog to investigate "third-party advertisement" contained in the NDP's 2012 leadership convention guide.

When asked about the ads contained in the guide, Rotman told CBC News that the NDP "did not charge any money" for them.

The ads are from various organizations including STRATCOM, The MPH Group, the Canadian Jewish Political Affairs Committee, MediaStyle, CAVAN Advertising Ltd., and the firm of Ryder, Wright, Blair & Holmes.

Diane Benson, a spokesperson from Elections Canada, told CBC News that as a matter of policy, the commissioner of Canada Elections does not comment on complaints it receives or possible investigations.

With files from The Canadian Press