NDP Leader Thomas Mulcair was in the hot seat as he faced questions over whether House of Commons staffers, paid for by tax dollars, performed partisan work from a Montreal office.

Speaking to the Procedure and House Affairs committee on Thursday, Mulcair was on the defensive, insisting that the NDP had not broken the rules when it came to parliamentary staffers working alongside those doing party work at the Montreal office.

“Every step of the way we've been open and transparent with the House of Commons, we've been open and transparent with this committee. We’ve respected the rules,” he told the committee.

Mulcair said it wasn't until April that the rules underwent a "substantive change," which no longer permitted parliamentary staffers to work in a space controlled by a political party.

Mulcair said the parliamentary staff is no longer working from the Montreal satellite office.

The Opposition leader said he turned over documents to the committee to show the office was paid for by party funds.

"Not a single cent of taxpayer money was used for renting those offices," he said.

The Liberals and the Conservatives are accusing the NDP of breaking the rules when it hired staff to work in Mulcair's satellite offices. Those staffers had been approved to do constituency work in Ottawa, but documents show they were working in a Montreal party office.

It's the first time in Canadian history that an opposition leader has been ordered to testify at a committee.

Mulcair, meanwhile, insists the party has done nothing wrong.

He said parliamentary and party staffers are in two separate unions and their work was kept separate.

"We are the only political party in parliament that is unionized," he said, noting that the collective agreements clearly outline the work of each group.

"It gives me the advantage to say that I know the two have always been separated," he said.

New documents obtained by CTV News on Thursday suggest that parliamentary staffers may have worked on the November byelection in the Montreal riding of Bourassa.

"Mulcair says they were only doing parliamentary work, but in the documents that CTV obtained, it suggests by virtue of looking at cheques and looking at invoices that some of these House of Commons staffers may have been working on the election campaign," CTV's Ottawa deputy bureau chief Laurie Graham told News Channel on Thursday.

The party says the House of Commons staffers were on a leave of absence during the byelection – which is permissible. The NDP also denies that the campaign was run out of the Montreal office.

Thursday's revelation comes on the heels of accusations that the NDP used taxpayer dollars to pay for 1.8 million partisan mailings letters sent to 26 ridings.

Mulcair said the accusations are false and the NDP followed the rules when it came to the mailings. However, on Wednesday, CTV News obtained a letter from Elections Canada sent last week to the Speaker of the House saying it is investigating the mailings.

In the letter, Elections Canada Chief Electoral Officer Marc Mayrand tells House Speaker Andrew Scheer: "These are complex matters that warrant careful consideration as they can have very significant repercussions on the activities carried out by Members of Parliament."

Mulcair said the party followed the rules in sending the mailings.

"It's completely legit, it's completely accepted, it's within the rules," Mulcair said.

The Liberal Party had previously complained to the Commissioner of Elections about mailings the NDP sent ahead of Bourassa federal byelection. Elections Canada later said the New Democrats did nothing wrong when they used their free parliamentary mailing privileges to send partisan flyers.

"With regards to Elections Canada, everything that they've ever looked (in regards to) what the NDP has done with these mailings shows the NDP respected the rules."

He said in the Bourassa case the accusations turned out to be false.

"And of course in this case it's completely false."

Board of Internal Economy letter