The Otago Regional Council is investigating what killed hundreds of fish in a Dunedin stream over the weekend.
The council's pollution team visited the site at Kaikorai stream and have taken water samples for testing.
Its regulatory general manager Richard Saunders said the most likely scenario was that warm temperatures and reduced dissolved oxygen in the water exacerbated the underlying water quality of the stream.
The opening of the coastal mouth at the Kaikorai estuary over the weekend was another potential contributing factor in the fish deaths as 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," 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."
The incident highlighted that water quality was everyone's responsibility, he said.
"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."
The Otago Fish and Game Council confirmed on social media on Sunday that hundreds of fish including smelt, flounder, bullies, trout and inanga had died and were littering the shoreline especially around the Brighton Road bridge.
"It's too early to speculate about a cause and while that's the case, we'd suggest not coming into contact with the Kaikorai estuary water, or the fish," the council said.