A striking red algal bloom drifted along the Illawarra coastline yesterday, forming spectacular flaming streaks off the region's beaches.
Thirroul Beach was closed for almost four hours, while reddish streaks also settled in waters off Puckeys Beach, Wollongong's Continental Baths and Flagstaff Hill.
By mid-afternoon Wollongong Harbour was home to a large, smelly pool of algae, stopping joggers in their tracks and causing a stir among passing school children.
Wollongong City Council beach services co-ordinator Jason Foye said the bloom had been hanging between one and two kilometres off the region's coast for the past few days.
"There's been a large amount of red algae out there and the wind's started pushing it back to shore," he said.
Outbreaks have plagued a number of Sydney beaches throughout the week.
Just how long the bloom might stick around is dependent largely on the weather and tidal conditions.
"These blooms have occurred as a result of prevailing weather and ocean conditions," a Metropolitan Sydney, South Coast and Hunter Regional Algal Co-ordinating Committees spokesman said.
"The blooms will dissipate naturally as conditions change."
Experts believe the algae is Noctiluca scintillans, also known as sea sparkle because it may appear to be phosphorescent at night.
Illawarra photographer Matt Smith captured the dazzling blue display off Stanwell Park on Wednesday night.
"[It was] the most amazing thing I've seen in ages, it was like a thunderstorm under the water with all the flashes," he said.
Although the algae has no known toxic effects, it can produce high ammonia levels that may cause skin irritation in some people.
Mr Foye urged swimmers and surfers to avoid direct contact with the algae, and to get out of the water and wash thoroughly if they did find themselves in the bloom.