An oil leak at the Tufts Cove power plant in Dartmouth, N.S., that spilled thousands of litres into Halifax harbour was much bigger than previously disclosed.

Nova Scotia Power has said the leak on Aug. 2 spilled about 5,000 litres into the harbour. But on Tuesday, the company said an additional 9,900 litres leaked into a containment trench and another 9,400 litres entered the cooling water system of one of its generators.

The utility estimates it will need at least another month to complete the cleanup.

"We expect mid-September this will all be cleaned up," said Mark Sidebottom, chief operating officer at Nova Scotia Power.

Mark Sidebottom is the chief operating officer of Nova Scotia Power. (Jean Laroche/CBC)

He said about 95 per cent of the oil that entered the harbour and containment pit has been recovered.

"And that happened quick, in the first several hours after the leak," Sidebottom said.

About 50 per cent of the oil in the cooling water system has been recovered. That work has been slower, according to the executive, because the focus of efforts has been on cleaning up what spilled into Tufts Cove.

Unknown why pipe failed

Sidebottom said about a dozen crabs and possibly one seagull have died as a result of the spill.

The oil came from a hole in a steel pipe, but the company still doesn't know why the pipe failed.

Sidebottom said it might take a couple of months before Nova Scotia Power knows for sure.

The company overseeing the cleanup, Eastern Canada Response Corporation, said there are about 70 people taking part in the operation.

Robert Starkes is the manager of Eastern Canada Response Corporation, the company overseeing the cleanup. (Jean Laroche/CBC)

The company's manager for the Atlantic region, Robert Starkes, said, "Nova Scotia Power has been extremely supportive in terms of the plans that we developed and presented."

He also said people will keep an eye on the area beyond the cleanup to ensure the oil is recovered.

"There will be a period of monitoring to ensure that there are no surprises," Starkes said. "So there will be folks doing surveys along the shore, perhaps aerial survey and that can be accomplished from the top of the building here. It's a great asset."