Mary Dawson told a Commons committee today she's referred the matter to Elections Canada, which is already considering Tory concerns.
Conservative MP Dean Del Mastro wrote Dawson asking her to investigate whether the NDP broke ethics guidelines during its spring convention in Vancouver.
The Tories say signs were posted during the NDP's policy convention last June that indicated unions were sponsoring various events.
"I can confirm that I received a letter with relation to that issue that did not satisfy the requirement for reasonable grounds for a request for an inquiry under the members' code," Dawson told the House ethics committee.
"I'm of the view that this matter may fall within the jurisdiction of Elections Canada and for this reason I forwarded the first letter that I received from Mr. Del Mastro to the commissioner of Canada Elections.
The Canada Elections Act prohibits unions from making political contributions.
Dawson says she requested and received further information on the matter from interim NDP Leader Nycole Turmel and she's now preparing responses to all interested parties.
"In light of my ongoing review of this matter and the confidentiality of my deliberations, I will have no further comments at this time."
Nevertheless, Del Mastro was dogged in his pursuit of the issue during the committee hearing.
"It would seem very clear to me that receiving tens of thousands of dollars for a political event would be an infraction of Section 14 of the code," he said, referring to part of the MPs' conflict code prohibiting gifts or benefits that could be seen as promoting undue influence.
Replied Dawson: "I'm listening to all your comments and I'm still considering the matter."
In a letter to Elections Canada written Aug. 31, Conservative party lawyer Arthur Hamilton said "it appears the NDP has received what the commissioner of Elections Canada has deemed to be contributions in contravention of the Elections Act."
"The Conservative Party of Canada would therefore request that your office take the steps it deems necessary to review this matter fully and identify any contributions which must be returned."
Hamilton attached several photos taken of signs and flyers from the convention.
One of the events that included a union sponsorship was a dinner featuring the late NDP leader Jack Layton and Nova Scotia Premier Darrell Dexter. The sign for the event carried the symbol of the Public Service Alliance of Canada.
A flyer for another $300-a-ticket "intimate reception" with Layton featured the logo of the United Steelworkers union.
Heather Wilson, the NDP's director of fundraising and membership, has said advertisements and sponsorships at "fair market value" are allowed by law and Elections Canada is aware of the practice.
"If there were penalties for frivolous and vexatious complaints, surely this would be one," New Democrat Pat Martin told the committee.
"I think Dean is far too good an MP to actually believe that this complaint was properly before the ethics commissioner. But he seems to be using this scattergun of stupidity lately."
Martin accused Del Mastro of "misusing the process just in order to grandstand on an issue that he knows full well is not properly there."
Earlier this year, four Conservative officials were charged with regulatory violations of the Canada Elections Act related to electoral overspending.
Under the so-called "in and out" scheme, the party transferred funds to dozens of riding associations and then directed them to send the money right back in order to pay for radio and TV advertising during the 2006 election.
Elections Canada argued the advertising was national in nature and resulted in the party exceeding its spending limits. The Conservatives disagree, and have been fighting Elections Canada in court.
The Federal Court of Appeals sided with Elections Canada and the Tories have filed an appeal with the Supreme Court of Canada.
The Conservative claim against the NDP generated an indignant response from the United Food and Commercial Workers Union.
The union's top Canadian official issued a news release calling it "a groundless, strategic assault on the labour movement" timed to coincide with Labour Day.
Mobile users, read Kady O'Malley's live blog of Thursday's Ethics committtee meeting here.