Picton harbourmaster Alex van Wijngaarden is calling for the person behind a source of pollution seeping into the harbour to own up.
At sporadic intervals, oil or diesel-based pollution is coming from the Kent Street stream, seeping into the Waitohi Stream and into the harbour.
The pollution is most noticeable with rain and council staff have been unable to find the source.
Captain van Wijngaarden said such toxic material can be fatal to wildlife. He said it's possible that it's coming from a small backyard operator who is oblivious to the damage being done.
Captain van Wijngaarden said the person or business responsible can expect to be prosecuted.