Statistics Canada says it is still following up to determine what caused a 45-minute service interruption to its census website Monday night.

Enthusiastic respondents who tried to fill out their census surveys online Monday evening were met with a notice saying the site was down for scheduled maintenance.

Census letters were sent out across the country yesterday that contained 16-digit access codes to allow people to fill out the mandatory surveys online.

The move to online census completion was aimed at making data collection more efficient and convenient. However, households trying to fill out their forms on Statistics Canada's census website found a message saying the site was undergoing "scheduled maintenance."

A spokeswoman for the agency confirmed the service outage in an email to CBC News

"On the evening of May 2nd, access to the census website was interrupted for a period of 45 minutes," Marie-Pier Desaulniers said.

A tweet from Statistics Canada suggested the system was overwhelmed by the large number of Canadians attempting to access the site.

The agency said it worked with Shared Services Canada to restore site access promptly, adding the census website will be monitored 24/7 throughout the collection period to ensure it keeps working.

"We thank Canadians for their enthusiasm in completing their census online," said Desaulniers.

However, when asked by CBC News in an email Tuesday if a high volume of participants was the reason for the problem, a spokesperson for Statistics Canada said only that the agency is "following up to determine the cause of the interruption."

Innovation, Science and Economic Development Minister Navdeep Bains said the outage was a sign of Canadians' enthusiasm for the census.

"It's great — a lot of positive feedback," Bains told CBC's Julie Van Dusen on Tuesday. "People are very excited about the census, and it's a positive response. People understand the importance of the census for building and communities and for planning."

As for the service interruption, Bains said, "It's a good problem, but we'll definitely fix it as soon as possible."