The Department of Fisheries and Oceans closed the Charlottetown Harbour on Monday. (Department of Fisheries and Oceans)

Charlottetown Harbour has been closed to shellfishing for at least seven days because of possible contamination caused by sewer overflow after heavy rain on the weekend.

'The systems were originally designed when they have those storm events to relieve themselves direct to the harbour.'— Craig Walker, City of Charlottetown

The problem came from an older part of the sewer system, where the storm and sanitary sewers are combined into one system.

"On a normal day, like today, all of the flows in that system would get collected," Craig Walker, manager of Charlottetown's sewer and water facility, told CBC News Tuesday.

"On a heavy rain event day, none of our treatment systems are designed to treat storm water volumes…. The systems were originally designed when they have those storm events to relieve themselves direct to the harbour."

Hundreds of oyster fisherman are affected by the closure.

The closure extends to just beyond the Hillsborough Bridge, a short distance up North River, up the West River to Drakes Point and to the Northumberland Strait. The Department of Fisheries and Oceans issued the closure order Monday.

Walker said a consultant is working on a plan to separate the sanitary and storm water sewers, but it would be an expensive project.

The provincial Department of Environment will be taking water samples, and the Canada Food Inspection Agency will be testing shellfish from the closed area.

The area will be reopened when fecal coliform levels are within approved limits.