OTTAWA -- Prime Minister Justin Trudeau appeared to grumble about NDP Leader Tom Mulcair in question period Monday, muttering about the NDP rejecting Mulcair and wondering whether he is the interim or outgoing party leader.

The grousing came at the end of the first question period after a two-week break, in which the opposition unrelentingly attacked the government over proposed changes to House of Commons rules and a controversy over Defence Minister Harjit Sajjan.

Mulcair took all the questions allotted to the NDP on Monday, rather than letting other New Democrat MPs pose any. Prime ministers sometimes respond to all questions from party leaders, but on Monday Trudeau responded to Mulcair's queries only in the leaders' round - the first set of questions posted by the opposition party leaders.

That led Mulcair to complain toward the end of the hour that Trudeau was breaching parliamentary tradition.

"Stephen Harper took all questions every day during the Senate scandal," Mulcair said, to loud applause from the opposition benches.

"The prime minister wants to change the fundamental rules of Parliament in order to help himself. And why all of this? Well, because he says he values question period and accountability. That's why he wants to scrap it. If that's true, why doesn't he stand and ask Canadians to listen to answers to some of our questions for once instead of his usual platitudes or non-answers?"

When Government House Leader Bardish Chagger rose to answer, Trudeau quietly heckled Mulcair, who last year lost a party confidence vote but is remaining NDP leader until the fall.

"It's amazing the NDP rejected him," says a voice that sounds like Trudeau (about 47 seconds into the video above).

Then, a few seconds later, he joked about Mulcair's position since the confidence vote.

"The outgoing leader? The interim leader?"

The Prime Minister's Office didn't deny he made the remarks and wouldn't say whether he was being sarcastic.

Asked about Trudeau's comments, Mulcair told CTV News that "today was not a good day" for the government.

"What you saw today was essentially that Justin Trudeau believes in transparency only when it suits his purpose," he added.

Trudeau has promised improved civility in the House, including discouraging Liberal MPs from heckling, as part of his "sunny ways" approach to government.

Even aside from Monday, the prime minister hasn't been immune to the desire to give voice to his frustration.

In 2011, the then-opposition MP referred to Conservative Peter Kent as a "piece of s---." Kent was then the environment minister and Trudeau said he was frustrated with Kent's "patronizing condescension" to opposition MPs.

Nearly a year ago, the prime minister inadvertently elbowed a New Democrat MP when he grabbed Conservative whip Gord Brown to drag him to his seat. Trudeau was frustrated that opposition MPs were delaying a vote amid a controversy over draconian rules proposed by his government. House votes can't happen until at least one party whip is seated.

Trudeau's spokesman said he had no comment on Trudeau's remarks. A government official noted there can be tense exchanges in the House on any given day, and that Monday was one of those days.