National Energy Board spokesman Darin Barter said the gas has been shut off and no one was hurt.
"The release is over. There is no public safety impacts. There is really no environmental impacts," he said Tuesday.
TransCanada (TSX:TRP) said the leak on a section of a branch line was detected Monday at around 10:42 p.m. in a rural area near the South Saskatchewan River — about 15 kilometres northeast of Medicine Hat.
The company said the nearest home is more than four kilometres away.
"TransCanada’s gas control centre detected a reduction in natural gas pressure on its Suffield Lateral line," said Davis Sheremata, a company spokesman.
"This section of 12-inch natural gas pipeline has been safely shut-in and the company’s emergency response procedures have been activated."
Details about the cause of the leak and how much gas was vented were not immediately available.
TransCanada, the NEB and the Transportation Safety Board are investigating.
"Gas detection equipment continues to confirm that conditions for TransCanada responders remain safe," Sheremata said.
He also said because of the leak shippers can't bring natural gas onto its Nova gas transmission system at six receipt points.
Other pipeline operations continue to operate normally and TransCanada continues to meet delivery requirements in the area, Sheremata said.
Sweet natural gas, also known as methane, does not contain significant amounts of hydrogen sulphide found in sour gas, which is toxic to people.