It could take weeks to identify the substance that turned a Queenstown creek neon green and flowed into Lake Wakatipu yesterday.
The Queenstown Lakes District Council said the green substance is similar to a dye used to map waste and stormwater flows, but obtaining conclusive results could take a long time.
The bright green liquid was spotted pouring from a stormwater water drain as well as flowing directly into Lake Wakatipu on Sunday.
Council contractors had traced the location of the source back to Horne Creek, near Queenstown's library.
CCTV footage showed the substance entering the creek about 2.28pm on Sunday.
The council's chief engineer, Ulrich Glasner, said the water was clear in this area around 10 minutes later.
"The substance does have similar characteristics to water tracing dye which is used for mapping waste and stormwater flows," he said.
"This is not a toxic substance but is certainly an unnecessary addition to an environment we work so hard to protect."
He said the incident was not connected with any council operations.
The council was continuing to review CCTV footage to investigate 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."
Tourist Georgia Rose said green water was pouring from a stormwater drain about 3pm on Sunday.
Shocked by what she saw, she said the water was going directly into Lake Wakatipu and slowly spreading across the beach and about 5m out into the lake.
The unusual sight had drawn the curiosity of a large number of tourists who had gathered around the lake edge yesterday after to capture what was going on.
"It's flocked hundreds of tourists to this end of the beach to take videos. There's quite a crowd."
The water was pouring out of a pipe near the children's playground by Queenstown Gardens, she said.
Curious as to why the water was green, she said they followed a stream back through the gardens to try to work out where it was coming from. That's when they spotted the drain.
"It's right on the lakefront at Lake Wakatipu."
"It's coming from the same body of water. We kind of followed it and it's coming through the stormwater drains, from the centre of the city, and it's just pouring into the lake right now.
"It's gone green, quite far down the beach."
She hadn't touched the water but said she hadn't noticed a peculiar smell.
"I haven't touched it actually because I wasn't sure what it was. I haven't noticed any smell, I'm not sure what it is."