It's not easy being green, but harbour spill no concern

A spill that turned parts of Sydney Harbour bright green has been found to be non-toxic, but the origin will likely remain a mystery.

Firefighters were called to the Milson Park bay in the harbourside suburb of Kirribilli on Wednesday afternoon after local residents observed that the water had turned neon green.

Rescue workers with the help of local council rangers and engineers plugged a drainage system with spill socks and were able to clear it of any traces of green water by early evening.

According to Fire and Rescue NSW Superintendent Adam Dewberry, the agency had determined the event had no negative environmental impact.

"I can confirm to you is that it's not toxic," the superintendent told AAP.

"I had a look down there this morning. There's no marine life that's floated to the water dead. There's no staining or marks on any of the wharfs or the boats that are in the bay."

Authorities believe the colour was probably caused by fluorescein, an odourless substance used to dye water in plumbing networks for diagnostic tests.

But because of the size of the area and the amount of potential sources Supt Dewberry said it was unlikely that they would be able to determine its origin.

"There's extremely old buildings in construction, there's new buildings, there's sewer systems or stormwater systems all throughout," he said.

"Was it a plumber, was it a prank? I don't know, just don't know."

Supt Dewberry said community members he interacted with showed little concern due to the lack of any signs of environmental damage.

Residents had observed the colour hours before emergency services were called.

"I would say there was more of an interest from the community, not an element of concern," he said.

"They went, 'oh, it's green'."