Neon 'green cordial' water in Sydney Harbour baffles locals

A bay in Sydney Harbour has turned neon green, with authorities urgently investigating the cause.

The bay along Hipwood Street in Kirribilli, near Milsons Park, turned bright green about 3pm, baffling locals who have called the incident "unusual".

Firefighters said they were "alerted to a large spill of fluorescent green material flowing into the harbour".

READ MORE: Man charged for allegedly keying Tesla at Woolworths carpark

One passerby described the water as "green cordial" and said it resembled "radioactive stuff you see in superhero movies".

NSW Fire and Rescue has run several tests and said the water is not toxic and doesn't appear to be harming the environment.

The source of the spill is still unclear, but they believe it was coming from a storm drain, they said.

READ MORE: East coast warned to expect heavy rain and plunging temperatures

They are now trying to flush it out and said the incoming rain should help turn the water colour back to normal.

A local told 9News they had seen green water in a stormwater drain at Anderson Park, along Clark Road, yesterday, before the green hue spread to the bay today.

Green water was also spotted at another park in Neutral Bay yesterday, another local told 9News.