ORC General Manager Regulatory Richard Saunders said there were a number of factors that could influence fish deaths in a case like this.

“The most likely scenario in this case is that warm temperatures and reduced dissolved oxygen in the water exacerbated underlying water quality issues in the stream, leading to the fish deaths."

Another potential contributing factor in the fish deaths is the opening of the coastal mouth, which occurred over the weekend. ORC’s Engineering Team opened the coastal mouth at the Kaikorai estuary when the stream reached a trigger point, at which it can cause water to flood and backfill into the Green Island leachate fields.

“Opening the coastal mouth is something ORC does regularly as a flood protection measure, and it’s essential to avoiding leachate from the Green Island landfill entering the stream,” Mr Saunders said.

“While opening the coastal mouth has not been associated with fish kills in the past, it is possible that it was an exacerbating factor in this fish kill event – that’s something we hope to understand from our investigation.”

Mr Saunders said this incident highlighted once again that water quality was everyone’s responsibility, in urban contexts as well as rural.

“Urban waterways have some of the most degraded water quality in New Zealand. It’s crucial that people are conscientious about what’s going into the streams and rivers in their backyards, and avoid draining anything that might negatively impact the environment.”