For the second time in a year, the New Democratic Party has run afoul of political financing laws.

Elections Canada has ordered the NDP to pay back money it collected from unions who sponsored events at the party's national convention last year.

The party, which has criticized the Conservative government for a lack of transparency, offered no details on how much it paid back or to whom.

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"As stated in the letter from Elections Canada, the New Democratic Party is in compliance with the Canada Elections Act," said Chantal Vallerand, the NDP's acting national director. "There will be no further comments on the matter."

The Conservative Party of Canada had lodged a formal complaint with Elections Canada following the convention, underlining that union and corporate donations are banned in Canada.

The NDP had argued that sponsorships are above board, as long as organizations are charged "market value" for advertising to a specific audience – in this case delegates at a convention. One event, a dinner with late leader Jack Layton and Nova Scotia Premier Darrell Dexter, was sponsored by the Public Service Alliance of Canada.

But in a June 1 response to the Conservative Party, deputy chief electoral officer Francois Bernier said Elections Canada considered those sponsorships donations.

"Subsequent to [Conservative Party lawyer Arthur] Hamilton's letter, the New Democratic Party was reminded of this position and has taken the required steps," Mr. Bernier wrote.

He referred to a section in the Canada Elections Act that specifies that ineligible contributions must be paid back to the donor or else to Elections Canada, who then transfers the money to the Receiver General of Canada.

"Given that the NDP had sponsorships from unions at their last convention, Canadians deserve to know what happened to these illegal donations," Conservative Party spokesman Fred DeLorey said. "How much money did the NDP rake in, and where is the money now?"

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Those details will emerge eventually when the party's financial filings are modified and posted by Elections Canada online.

The Commissioner of Canada Elections slapped the NDP on the wrist last fall after investigating how the party solicited funds on behalf of another entity – a no-no under Canadian law.

The NDP had taken in donations to commemorate Mr. Layton's death to be passed on to the new Broadbent Institute think tank. It also promised to issue tax receipts. Ultimately, it had to return money to donors and set up an another system for contributions to be routed to the institute.

The Conservatives have also come under the Elections Canada microscope.

In March, the party paid back Elections Canada $230,198 and dropped an appeal at the Supreme Court. The case involved the funnelling of national funds to local ridings that in turn paid for national advertising in the 2006 election – a practice that caused the party to exceed its legal spending limits.

The party also settled last November for another $52,000 over a separate set of regulatory charges related to the so-called "in and out" scheme.

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The electoral watchdog has also been probing allegations of voter suppression during the last general election. The Conservatives have acknowledged some irregularities in one Ontario riding, but there is no evidence directly linking the party to misleading "robo-calls."