The Liberal government is reinstating the mandatory long-form census that was scrapped by the Conservatives five years ago.

"We need good, reliable data," said Minister of Innovation, Science and Economic Development Navdeep Bains, who made the announcement on Parliament Hill.

Bains could not say what the penalty would be for refusing to fill out the compulsory questionnaire, but said there will be a "robust" communication plan. The vast majority of Canadians understand the importance of this data and want to participate in the process, he said, noting that 93.5 per cent of the population filled out the forms last time.

The data is invaluable for everyone from city planners and provincial governments to businesses and non-government organizations, Bains said.

Minister of Families, Children and Social Development Jean-Yves Duclos said Canadians are happy to fill out the long form because they understand it is critical to the well-being of the country.

A promise to "immediately" restore the long-form census was one of the planks in the Liberal Party's platform during the recent federal election.

Today's announcement comes a day after Prime Minister Justin Trudeau and his new cabinet was sworn in.

"We committed to a government that functions based on evidence and facts and long-form censuses are an important part of making sure we're serving constituents in our communities," Trudeau said, pausing to take a question as he sprinted up the steps to his office on Parliament Hill.

Prime Minister Trudeau brings back the long form census0:37

New census packages must be ready to mail out by May 2.

The long-form census would have to be distributed to about 2.9 million households.

The Conservative government cancelled the mandatory long census form for the 2011 census, replacing it with a voluntary national household survey. Bains said that was a bad move based on ideology.

"We know the history of the past government and they very much focused on ideology," he said. "We're focused on sound, evidence-based policies. We want to make sure we're driving good policies based on good evidence and quality data."

'Broader review' better

Conservative MP Tony Clement, who was industry minister when the decision was made to scrap the long-form census, said in hindsight he would have done things differently. Going forward, the Liberals would have been better to carry out a "broader review" of data collection. He said there are other ways to capture data that protects the privacy and security of Canadians.

"Other countries are moving away from traditional census-taking and moving towards the data collection on a broader scale to get the data that's necessary for researchers, for businesses and academics," he said.

John Campey, who was part of the Save the Census campaign, said the detailed information is a critical "building block" for community programs. He said the cancellation was a "disaster" for gaining key demographic information to tailor services effectively.

"It's critically significant both symbolically and practically; it's one of those things that cuts both ways," he said.

Campey said there will be a "gap" from the 2011 data that can never be filled.

All Canadians were to still receive a mandatory short census form, with one in three households sent the new household survey as well. Before it was scrapped, one in five households were sent the mandatory long census form.

The cancellation was widely criticized by researchers, analysts and planners who rely on high quality, detailed data for their work.

With files from CBC's Julie Van Dusen