what is the cause of that?
The flags are back up at Bondi Beach but thick red algal blooms are continuing to appear at beaches across Sydney's east.
A bloom was first spotted at Bondi at 6.30am (AEDT) on Tuesday, with subsequent outbreaks popping up at Clovelly Beach and Gordons Bay later in the day.
On the NSW central coast, blooms were spotted off Wamberal, Copacabana, Terrigal, Avoca and North Avoca beaches at around 9.30am but dispersed shortly after, a Gosford City Council spokesperson said.
No beaches were closed on the central coast.
While Gordons Bay and Clovelly, which resembled tomato juice on Tuesday afternoon, remain closed, Bondi Beach has reopened.
"With the red algae leaving and mostly harmless, Bondi is putting the flags back up again. Enjoy!" Beachwatch wrote on Twitter.
The bloom has been identified as Noctiluca scintillans, is caused by an upwelling of colder nutrient-rich water.
Also called "red tide" or "fire in the sea", the algae is non-toxic but contains high ammonia levels that can cause minor skin irritation if you swim in it, a NSW Office of Water spokesman told AAP.
He said the blooms were a common natural phenomena and more likely to occur in spring and autumn when there were higher water temperatures and more movement in ocean currents.
"(It) can also often be seen after rainfall events in the vicinity of river mouths."
At night, the algae can appear phosphorescent.
Waverley head lifeguard Bruce Hopkins said the bloom had "quite a fishy smell to it", had a "reddy-purple" tinge and sits on the surface like oil sheen.
"It can irritate some people's skin but generally not much more than that," he told AAP.
Red algae was uncommon but not unheard of at Bondi, Mr Hopkins added.
People are advised to avoid swimming in discoloured water.
It is not known when the Clovelly or Gordons Bay will reopen.