Prime Minister Justin Trudeau is under pressure to hold a national referendum on electoral reforms, but he will only commit to a broad, cross-party consultation process.

During question period Wednesday, interim Conservative Leader Rona Ambrose warned Trudeau to be "very careful" in assuming his election victory gives him a mandate to make significant democratic changes. She demanded Trudeau hold a referendum before making "fundamental change" to Canada's voting system.

"When you change the rules of democracy, everyone gets a say," she said.

Trudeau said Canadians have long been frustrated with a process that "disengages" voters, and promised to move to a system that better reflects the concerns and priorities of Canadians.

"We will engage as promised in broad consultations with Canadians," he said, firing back that the Conservatives had a record of refusing to listen to Canadians.

The Liberal campaign platform promised the 2015 election would be the last conducted under the first-past-the-post voting system. It pledged to strike an all-party parliamentary committee to explore a "wide variety of reforms, such as ranked ballots, proportional representation, mandatory voting and online voting."

Liberals would benefit from preferential ballots

Trudeau favours a preferential ballot system. A CBC News analysis of October's federal election results showed the system that is the party's preferred option would have delivered an even bigger victory for the Liberals.

That committee is to deliver recommendations to Parliament, with legislation to enact electoral reform promised within 18 months of forming government.

On Tuesday, Minister of Democratic Institutions Maryam Monsef said she did not want to "prejudice" the electoral reform consultation process, but promised it would be "open and robust" and led by an all-party committee.

Alexandre Boulerice, the NDP's deputy critic for ethics and democratic reform, said there are many ways to engage people in the process, and a referendum is one of them. But the end goal should be proportionality, he said.

"Every vote must count. We don't need these phony majorities elected with less than 40 per cent support," he said. "We don't need a system that exacerbates regional differences and fails to reflect the true will of Parliament.‎"