An urgent investigation is under way after a bay in Sydney Harbour turned “radioactive” green.

North Sydney Council says it is investigating after the water in the bay at Kirribilli, near Milsons Park, turned bright neon green about 3pm on Wednesday.

Firefighters said they were “alerted to a large spill of fluorescent green material flowing into the harbour”, while baffled locals said the incident was “unusual”.

One local told the Nine Network the water looked like “green cordial”, and resembled “radioactive stuff you see in superhero movies”.

Fire and Rescue NSW Superintendent Adam Dewberry said the green liquid appeared to have entered “our beautiful Sydney Harbour” through the drainage system at Kirribilli. He said because the area was old and at the bottom of a hill, there were “thousands of areas where [the substance] could have entered”.

“It’s unlikely we’ll be able to identify the source, where it did come from, because there’s just so many access points into the storm water … in that area,” he said.

“The main thing is we’ve identified it’s non-toxic … There’s no evidence of any marine damage, fish kills, [and] there’s no staining on any of the boats that are in the bay there, or around the wharves or the shoreline.”

Dewberry said there was no vapour or odour from the water, and it wasn’t acidic or alkaline.

“So there is no corrosiveness in it. We are not seeing any evidence of fish kills or damage to the environment,” he told The Australian.

North Sydney Council said rangers had visited the site on Wednesday, putting spill socks into the stormwater channel to stop further pollution from entering the bay.

Council engineers believe the substance is fluorescein – a chemical used by plumbers to dye water and find leaks.

The drainage network was clear of the green water by 6.40pm on Wednesday, while there were further attempts early on Thursday to flush out the substance.

Steady rain in Sydney in coming days was also likely to help the water return to a more normal colour.