Sydney residents have been left shocked and confused after witnessing seawater in one of the city's harbours turn blood red.

Locals passing by Careening Cover, which flows through Milson Park in Kirribilli, a wealthy Australian suburb, spotted the scarlet stream as they walked along the coast. The shock of red has left both locals and experts alike puzzled, and both local and regional officials sent to investigate.

North Sydney Council said it has sent officers to investigate the water, with the New South Wales Environmental Protection Authority (EPA) now also involved. But neither the government nor the agency has been able to trace the source of the dramatic colour, with only one theory having been floated so far.

Officials believe the red water may have been caused by plumbers' dye (

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After the issue was reported to the council, News.co.au reported that officials said they said they couldn't yet conclude where it came from, but that they suspected the change in colour was likely due to plumbers' dye. Plumbers' dye, also known as drain tracing dye or fluorescein, is used by plumbers to detect the flow of water in drains or pipes, or trace leaks.

A spokesperson for the council said: "Council is aware of the red water flowing into the harbour from Milson Park, and officers have investigated but have not been able to find the source." The NSW Environmental Protection Authority said in its own statement that it is investigating the "potential source".

The spokesperson said: "North Sydney Council alerted us to what appears to be red plumbers’ dye in a drain at Kirribilli this afternoon and we are currently investigating the potential source." They added that Fluorescein is not a toxic substance, but that it can create dramatic colours.

They said: "Fluorescein has low toxicity but even small amounts can be highly visual and for this reason its use should always be minimised." While bizarre, this isn't the first time Australians have seen their seasides tinged a deep red, with a similar selection of incidents having occurred more than a decade ago.

In 2012, several Australian beaches turned red due to algal blooms named noctiluca scintillans also known as "red tide" or "fire in the sea" washing up on several stretches of sand near Sydney. Much like the latest case of plumbers' dye, the algae was also thought to be non-toxic.

But ultimately, officials decided to ban people from swimming in the water while the blooms - which can come in both green and red, and may even glow in the dark - remained. Scientists believed the bloom was caused by local higher than usual water temperatures and increased movement in ocean currents.