The National Energy Board, an independent regulator, said first responders, TransCanada officials and members of its team were assessing a release of gas from the Nova Gas Transmission system.
The pipeline was shut down, no injuries were reported and the NEB said there were no immediate safety concerns for the public.
TransCanada said Tuesday it was working to remedy the situation as soon as possible.
Any repair work and restoration of service must be approved by the regulator.
The NEB said an energy company must protect the public and the environment in dealing with such an emergency.
"This includes cleanup and site remediation," the regulator said Tuesday. "We do not relax our expectations of a company, or our response, regardless of the size of the incident."
There was no update on the status of the line other than a "red light" posted Wednesday on TransCanada's website for the pipeline, a 15,000-mile network.
The NEB said it wasn't able to determine the size of the leak.