Sewage effluent poisons 13 diners

The norovirus outbreak has been linked to oysters contaminated by a sewage spill in the Coromandel area. Photo / Thinkstock

Damage to a previously "unknown subsurface drain" caused the spill of partially-treated sewage effluent that poisoned 13 people with norovirus when they ate oysters.

Effluent from the sewage treatment plant serving Coromandel township leaked into a stream and flowed into the sea, contaminating an oyster farm.

Oysters from the farm were served at a catered event in Auckland and at a Cambridge restaurant. Fourteen people at the Auckland event ate the oysters, of whom 10 became sick with norovirus. Three in the Cambridge incident ate oysters and all fell ill. Two Cambridge diners ate raw oysters. The third ate some undercooked oyster meat, before sending the meal back to be re-cooked.

The outbreaks, which occurred in 2009, are reported in today's New Zealand Medical Journal. The report says the leaking pipe was the "obvious source" of the contamination.

The Thames-Coromandel District Council, which owns and operates Coromandel town's wastewater infrastructure, said today the source of the leak was an "unknown subsurface drain" at the sewage treatment plant.

The drainage pipe, which was not a sewer, was located beneath a wastewater pond but was not part of the treatment plant's operating system.

"It's existence and purpose was unknown to the council. The pipe was damaged in excavation within the pond, causing the leak and disposal to a nearby stream, but this was underwater and it was unknown at the time."

Thirty-three days elapsed before the damage was discovered, after which it was repaired within 24 hours. Staff had noticed staining in the stream, 50m upstream of the treatment plant's outlet.

"Since the event, council has developed a methodology for undertaking environmental risk analysis and audits of its wastewater treatment plants. This is a continuing project to ensure a rigorous approach to risk management is applied."

The district council drew attention to the fact the incident had not led to its being prosecuted by the Waikato Regional Council.