It could take weeks for officials to work out what stained one of Queenstown's most iconic lakes neon green.
The water was clear in Lake Wakatipu 10 minutes after it was spotted turning green, an engineer says. (file pic) Photo: RNZ / Belinda McCammon
The vividly green water flowed down Horne Creek yesterday afternoon, snaking its way through central Queenstown and into Lake Wakatipu.
Water samples have since been taken to figure out the source.
Staff at The Bathhouse cafe, bar and restaurant on the shores of Lake Wakatipu described the vibrant green water as like a highlighter.
Sam - who was on a day off - said her work group chat lit up with photos of the strange scene.
"They saw the green water washing up against the shore. The boss asked where it came from and one of the girls went up to the stream ... near the playground and that's where they saw it coming from, but they couldn't go any further because they were working."
But there were no signs of the green this morning, and she said the nearby ducks all seemed fine.
CCTV footage showed the substance entering the creek shortly before 2.30pm near the library.
Queenstown Lakes District Council chief engineer Ulrich Glasner said the water was clear at that location roughly 10 minutes later.
"There is a lot of speculation as to what the substance is. Comprehensive testing is underway and it could take a matter of weeks to have a conclusive result," he said.
"The substance does have similar characteristics to water tracing dye, which is used for mapping waste and stormwater flows. This is not a toxic substance but is certainly an unnecessary addition to an environment we work so hard to protect.
"We are reviewing further CCTV footage and following up on all possibilities regarding where the substance originated from.
"We don't have any further information on that aspect, or what further action will be taken at this stage. We are taking the matter very seriously and would welcome any information which would allow us to identify the source of pollution."
Risk to native species - Forest and Bird
Forest and Bird fresh water advocate Tom Kay described photos of the vividly green waterways as striking and concerning.
"When we're putting the wrong kind of things in our stormwater, those things pollute the places where our fish and invertebrates and birds live. This is a prime example of something that shouldn't be ending up in Lake Wakatipu, ending up there and there's a risk for the native species that live there," Kay said.
"The investigation is critical with things like this. We have a lot of pollutants that get into our lakes and our streams and rivers from stormwater and wastewater drains that are leaking or have the wrong things put in them so it's critical to know where this has come from.
"I'm hopeful that it's not going to have an ongoing effect, but we just have to wait and see what that investigation finds out."
It was a prime example of why Aotearoa needed to improve its stormwater and wastewater treatment, he said.
"Overseas countries are looking at treating stormwater in the same way they treat wastewater because there can be serious pollutants in it and in New Zealand, all our stormwater just runs straight down into a stream or into a lake, into the ocean. We need to do much better."
The Otago Regional Council said it was too early to speculate whether the bright green substance did any environmental damage and it would decide on its next steps after receiving test results.
NIWA freshwater ecologist Tracey Burton said testing was the only way to confirm the source, but it appeared it could be rhodamine dye, which was commonly used to determine the direction, flow and transport of water.
It's also been used by plumbers to determine where leaks are in pipes.
If that was confirmed, she said it was approved for use in water.