The leak, near Red Earth Creek, Alta., was reported to the AER last Thursday.
"Cleanup activities are well advanced, with the majority of the work complete, and we will continue to ensure that we mitigate and manage any environmental impact," CNRL spokeswoman Julie Woo said in an emailed statement.
"Canadian Natural is working together with the Alberta Energy Regulator while we continue to investigate the cause of the incident."
The company and regulator say there was a "mechanical failure" on the pipeline in an area some 375 kilometres northwest of Edmonton.
Woo says the leak has stopped and most of the product was contained on the company's land, with some ending up along a nearby pipeline right-of-way.
The AER says there have been no reports of harm to wildlife.
In April, a pipeline owned by the company spilled 70,000 litres of oil and processed water in the region. The AER said at the time that the spill northwest of Slave Lake was not near any people, water or wildlife.
The AER is also investigating bitumen leaks at CNRL's Primrose oilsands project in eastern Alberta, which were discovered during the spring of 2013. Some 1.2 million litres of a bitumen-water emulsion have been recovered.
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