Surf's up! But wait, what's that red stuff?

No, it's not ketchup and Jaws hasn't attacked - it's just algae.

The marine plant managed to close Australia's world-famous Bondi Beach, as well as Sydney's Clovelly Beach.

If you want to cool off, the algae, known as Noctiluca scintillans or sea sparkle isn't toxic - but it can irritate the skin.

Local lifeguard Bruce Hopkins said it made the water look like "tomato sauce."

Irene Eristian, 33, told News.com.au the water colour was "intense." "I wasn't sure if I should let my daughter into the water as I wasn't sure what it was."

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• AUSTRALIA-NATURE-ALGAE

  A swimmer swims in the safety of the pool as a red algae bloom discolours the water at Sydney's Clovelly Beach on November 27, 2012, which closed some beaches for swimming including Bondi Beach for a period of time. While the red algae, known as Noctiluca scintillans or sea sparkle, has no toxic effects, people are still advised to avoid swimming in areas with discoloured water because the algae, which can be high in ammonia, can cause skin irritation. AFP PHOTO/William WEST (Photo credit should read WILLIAM WEST/AFP/Getty Images)

• AUSTRALIA-NATURE-ALGAE

  A swimmer stops short of a red algae bloom at Sydney's Clovelly Beach on November 27, 2012, which closed some beaches for swimming including Bondi Beach for a period of time. While the red algae, known as Noctiluca scintillans or sea sparkle, has no toxic effects, people are still advised to avoid swimming in areas with discoloured water because the algae, which can be high in ammonia, can cause skin irritation. AFP PHOTO/William WEST (Photo credit should read WILLIAM WEST/AFP/Getty Images)

• AUSTRALIA-NATURE-ALGAE

  A swimmer heads towards a red algae bloom at Sydney's Clovelly Beach on November 27, 2012, which closed some beaches for swimming including Bondi Beach for a period of time. While the red algae, known as Noctiluca scintillans or sea sparkle, has no toxic effects, people are still advised to avoid swimming in areas with discoloured water because the algae, which can be high in ammonia, can cause skin irritation. AFP PHOTO/William WEST (Photo credit should read WILLIAM WEST/AFP/Getty Images)

• AUSTRALIA-NATURE-ALGAE

  A swimmer swims in the safety of the pool as a red algae bloom discolours the water at Sydney's Clovelly Beach on November 27, 2012, which closed some beaches for swimming including Bondi Beach for a period of time. While the red algae, known as Noctiluca scintillans or sea sparkle, has no toxic effects, people are still advised to avoid swimming in areas with discoloured water because the algae, which can be high in ammonia, can cause skin irritation. AFP PHOTO/William WEST (Photo credit should read WILLIAM WEST/AFP/Getty Images)

• AUSTRALIA-NATURE-ALGAE

  A boy walks pas a red algae bloom discolouring the water at Sydney's Clovelly Beach on November 27, 2012, which closed some beaches for swimming including Bondi Beach for a period of time. While the red algae, known as Noctiluca scintillans or sea sparkle, has no toxic effects, people are still advised to avoid swimming in areas with discoloured water because the algae, which can be high in ammonia, can cause skin irritation. AFP PHOTO/William WEST (Photo credit should read WILLIAM WEST/AFP/Getty Images)

• AUSTRALIA-NATURE-ALGAE

  A swimmer heads towards a red algae bloom at Sydney's Clovelly Beach on November 27, 2012, which closed some beaches for swimming including Bondi Beach for a period of time. While the red algae, known as Noctiluca scintillans or sea sparkle, has no toxic effects, people are still advised to avoid swimming in areas with discoloured water because the algae, which can be high in ammonia, can cause skin irritation. AFP PHOTO/William WEST (Photo credit should read WILLIAM WEST/AFP/Getty Images)

• AUSTRALIA-NATURE-ALGAE

  People walk past a red algae bloom discolouring the water at Sydney's Clovelly Beach on November 27, 2012, which closed some beaches for swimming including Bondi Beach for a period of time. While the red algae, known as Noctiluca scintillans or sea sparkle, has no toxic effects, people are still advised to avoid swimming in areas with discoloured water because the algae, which can be high in ammonia, can cause skin irritation. AFP PHOTO/William WEST (Photo credit should read WILLIAM WEST/AFP/Getty Images)

• AUSTRALIA-NATURE-ALGAE

  A swimmer swims in the safety of the swimming pool as a red algae bloom discolours the water at Sydney's Clovelly Beach on November 27, 2012, which closed some beaches for swimming including Bondi Beach for a period of time. While the red algae, known as Noctiluca scintillans or sea sparkle, has no toxic effects, people are still advised to avoid swimming in areas with discoloured water because the algae, which can be high in ammonia, can cause skin irritation. AFP PHOTO/William WEST (Photo credit should read WILLIAM WEST/AFP/Getty Images)

• AUSTRALIA-NATURE-ALGAE

  A seagull stands a red algae bloom discolouring the water at Sydney's Clovelly Beach on November 27, 2012, which closed some beaches for swimming including Bondi Beach for a period of time. While the red algae, known as Noctiluca scintillans or sea sparkle, has no toxic effects, people are still advised to avoid swimming in areas with discoloured water because the algae, which can be high in ammonia, can cause skin irritation. AFP PHOTO/William WEST (Photo credit should read WILLIAM WEST/AFP/Getty Images)

• AUSTRALIA-NATURE-ALGAE

  People inspect a red algae bloom discolouring the water at Sydney's Clovelly Beach on November 27, 2012, which closed some beaches for swimming including Bondi Beach for a period of time. While the red algae, known as Noctiluca scintillans or sea sparkle, has no toxic effects, people are still advised to avoid swimming in areas with discoloured water because the algae, which can be high in ammonia, can cause skin irritation. AFP PHOTO/William WEST (Photo credit should read WILLIAM WEST/AFP/Getty Images)

• A mother and her daughter check out the red algal bloom in the water at Clovelly Beach in Sydney Red algae closes beaches in Sydney, Australia -

  A mother and her daughter check out the red algal bloom in the water at Clovelly Beach in Sydney Red algae closes beaches in Sydney, Australia -

• A mother and her daughter check out the red algal bloom in the water at Clovelly Beach in Sydney Red algae closes beaches in Sydney, Australia -

  A mother and her daughter check out the red algal bloom in the water at Clovelly Beach in Sydney Red algae closes beaches in Sydney, Australia -

This isn't the first time water has turned blood-red this year. In August a lake in the South of France made the headlines after changing colour, but experts said the phenomenon was "completely natural".

Patricia Estebe from Camargue's tourist office explained: "This phenomenon is a result of the salt content in brine shrimp Artemia salina and algae Dunaliella salina. When the salt concentration is very high - which is the case before harvest - the brine shrimp die and saline algae proliferates giving this unusual colour. "

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