A murky brown wave that crept into Wellington's Evans Bay this morning and turned the bay's waters dark has been captured on video.
A Wellington City councillor said the substance was silt that had travelled from the Hutt River, after being stirred up by the weekend's heavy rainfall.
Andrew Jones watched the brown film flow into Evans Bay around 11.45am - and initially feared it could be sewage or some toxic spill.
"I looked out the window and there was this red-brown shadow ... creeping through Evans Bay, insidiously taking over [the bay]," he said.
Eastern Ward councillor Sean Rush said every time there was a downpour, the Hutt River and associated water channels were effectively flushed into the harbour.
"The visual is a clean line as depicted in the image and usually does shock people. Often they believe [it is] sewage. If it was choppy, as yesterday, you would not see it so well," Rush said.
The brown silt would be gone in two to three tide cycles, he said.
Detectives flew to Victoria, NSW and Queensland as part of the investigation.