Liberal MP Marc Garneau says he believes Elections Canada is investigating some 1.8 million pieces of mail sent out by the NDP to determine whether there was an abuse of the so-called franking privileges, which allow MPs to mail postage-free material to constituents.

Garneau told CTVNews.ca that he met with senior Elections Canada investigators in late April after lodging a complaint about the NDP's mailings.

The Liberal Party believes the mailings were "electoral in nature" and therefore should not have been sent out using free postage privileges, he said.

MPs are allowed to send mail postage-free, but they are prohibited from using parliamentary resources for electoral campaigning.

In his initial letter of complaint, Garneau asked Elections Canada to investigate the NDP for allegedly using franking privileges to send mail to voters in the Montreal-area Bourassa around the time of the November 2013 byelection.

Garneau has said that if the mail was delivered after the byelection was called, the value of the mailings would be considered illegal donations to the NDP candidate.

The Liberals raised similar concerns regarding NDP mailings in the Manitoba ridings of Brandon-Souris and Provencher, as well as Toronto Centre.

But the NDP said the mailings were sent out before the writ period began and therefore could not be counted as election expenses. The party has maintained that it did nothing wrong.

On April 1, the NDP issued a news release saying it has been cleared by Elections Canada.

After the all-party Board of Internal Economy asked Elections Canada to look into the matter, Chief Electoral Officer Marc Mayrand responded with a letter that said, in part: "In the case of mailings sent by a member of Parliament, Elections Canada's long-standing position is that when a mailing is issued prior to the writ, or when the mailing is in transit when the election is called and the member is unable to stop the delivery, the mailing is not a regulated expense."

But Garneau said the Liberals have asked Elections Canada to look into the bigger issue of about 1.8 million pieces of NDP mail that may be "electoral in nature" and therefore should not have been sent out using taxpayer money.

He also alleges that the flyers were produced on a non-government printer.

After meeting with Elections Canada investigators on April 29, Garneau said he was given the impression that Elections Canada is looking at "all the mailings." He also said the officials told him they planned on meeting with the House of Commons’ legal services that same day.

"It was certainly our request that they examine the whole group of mailings and not just those that went to the four byelection ridings," he said.

In an email to CTVNews.ca, Elections Canada spokesperson John Enright declined to comment on the issue, stating that it is not the policy of the commissioner to confirm or deny whether complaints have been received or if investigations are underway.

The NDP also declined to comment about Garneau’s allegations.

Garneau said his party has flagged 26 separate groups of NDP mailings, which amount to 1.8 million pieces of mail.

"If you had to put a stamp on each one instead of using franking privileges it would cost about $1.3 million," he said.

"In our opinion, these mailings … are electoral in nature, in which case they're an unlawful use of MPs mailing privileges and franking privileges for electoral purposes."