Oamaru Creek is still stained a brick red and the Otago Regional Council suspects it is being caused by rotting seaweed washed into the lagoon during recent heavy seas.
The council has inspected the creek after a complaint to its Pollution Hotline, but has not tested the water.
After a photograph of the red creek was published in Wednesday's Otago Daily Times, two people contacted the newspaper to say the colour was from rock which contained ferrous oxide being used to protect the foreshore opposite the Oamaru railway station.
However, there is little evidence of that at the foreshore and Oamaru Creek mouth.
Along the edge at the mouth, patches of the red seaweed could be seen. Council compliance manager Martin King said if the discolouration was coming from the sea, it would be visible in the seepage through the foreshore shingle bar.
"We think it is rotting seaweed releasing a red dye," he said.
The council was not planning to test the water at this stage, unless aquatic life was affected, he said.
