The Energy Resources Conservation Board said the spill was first reported by Apache Canada Ltd. on June 1.
The pipeline breach is about 20 kilometres northeast of Zama City, a remote community near the Northwest Territories boundary.
"The affected area is undergoing cleanup, environmental monitoring, wildlife protection and remediation efforts and is currently estimated at 42 hectares," the company said in a release Wednesday.
Apache said the nearby Zama River has not been affected by the leak, which has been plugged.
An Apache official declined to answer questions about the Zama spill.
Waste water that is extracted during oil and natural gas operations contains oil, gas, salt and other minerals.
Bob Curren of the resources conservation board said Alberta regulators didn't learn how big the spill was until Tuesday.
"At the outset we were unaware that it was of this extent or volume," Curren said from Calgary. "If we had known that up-front we would have made the announcement at that time.
"Once it was determined that the volumes were at this level we immediately moved to issue a news release."
Greenpeace Canada said the area of the spill is an important wetland and habitat for animals and birds.
The environmental group calls the leak one of the largest of its type in Canadian history.
Keith Stewart, a Greenpeace spokesman, said this kind of waste water is full of toxic compounds.
"This is a massive spill of toxics into one of the most important wetlands in Canada, if not the world," he said from Toronto. "The government shouldn't be trying to hide these kinds of things."
Greenpeace called on the Alberta government to release the findings of a pipeline safety review that was completed last year.
The province commissioned the report following spills from oil pipelines owned by Plains Midstream Canada in central Alberta last June and in April 2011 in north-central Alberta.
The Apache website says the corporation has oil and natural gas operations around the world including the Gulf Coast, the Gulf of Mexico, Egypt, the North Sea, Australia and Argentina with assets in 2011 worth US$52 billion.
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