The investigation into what went wrong with the Halifax sewage treatment plant is expected to cost more than $400,000.

Halifax Mayor Peter Kelly disclosed the amount Tuesday after regional council held a closed-door session to discuss the broken $54-million plant.

Kelly said U.S.-based CH2M Hill has been hired to conduct the forensic audit — the process of extracting the information from the facility's black box.

"We want them to do a thorough analysis," he told reporters.

The plant near the naval dockyard malfunctioned in January following a power outage. Since then, about 80 million litres of raw sewage has been flowing into Halifax harbour every day.

The removal of several screens at sewer outfall stations along the Halifax waterfront has compounded the stinky, unsightly problem.

Municipal officials said the screens had to be taken out because they were never designed to work continuously. As a result, sewage solids, known as floatables, have been washing ashore.

The Halifax plant isn't expected to be permanently fixed until next spring. One councillor told CBC News they were told Tuesday of a possible temporary solution to the sewage problem.

Jennifer Graham, with the Ecology Action Centre, isn't impressed with the secrecy.

"The Halifax harbour is a public asset," Graham told CBC News. "We are all concerned and interested in what's going on, and the public has a right to know. So I guess I'm wondering what's so secret that the public and the media can't be at this in-camera session."

Kelly said council had to discuss legal issues in private.

He said the results of the forensic audit should be ready in a couple of weeks and he hopes the $400,000 bill for the investigation will be paid for by the municipality's insurance company.

However, he added, taxpayers could still be on the hook for some costs.

"If there's a design flaw we'll go after the appropriate party to fix that issue. But if we're going to do enhancements to what's already there, then there may be costs incurred," Kelly said.

The Halifax plant is a key part of the $332-million Halifax Harbour Solutions project. After it opened, the mayor celebrated the drop in bacteria counts by reopening public beaches last summer and taking a dip in the water.

The Dartmouth sewage treatment plant is operating. A third plant in Herring Cove is scheduled to start running this summer.