Wellington City Council is pleading with residents to ensure only water enters stormwater drains after the Botanic Garden duck pond turned vivid white last week.
Councillor Ngaire Best said the colour change was probably caused by someone in the nearby suburbs of Northland or Kelburn washing paintbrushes and letting the runoff get into the stormwater drain.
The duck pond is fed by two streams, and stormwater drains from the suburbs feed directly into them.
"Street-side drains or sumps are direct links to our waterways," Ms Best said.
"No treatment, no filtering - nothing happens between what goes in the gutter and what ends up in streams and the sea."
The streams go through the duck pond and eventually feed into the harbour near Westpac Stadium.
"So the message is please, people, don't put anything down there you wouldn't want your children swimming in," Ms Best said.
Painting equipment should be washed in the laundry sink and cars washed on grass or at a carwash, she said.
Anyone caught polluting waterways - including by tipping anything other than water down streetside drains - could be fined thousands of dollars.