Surf Life Saving NSW CEO Steven Pearce said Austinmer Beach was a particularly popular swimming destination, along with many other Illawarra beaches.

He said an amber alert was issued on the Beachsafe website and app for Austinmer Beach due to overcrowding.

Read more: COVID rules for Wollongong over New Year's Eve announced

The alert advised swimmers the beach was starting to reach the maximum number of patrons allowed under the COVID-safe gathering restrictions, which is determined by Wollongong City Council's management plan.

The alert is designed to encourage people to attend a different beach.

Mr Pearce said lifesavers and lifeguards monitored the numbers, however the beach did not reach the maximum capacity.

Illawarra Surf Life Saving duty officer Anthony Turner said lifesavers reported a concerning trend of hundreds of people swimming outside the flagged locations over the holiday period, which resulted in multiple rescues by lifesavers, lifeguards and members of the public.

Mr Turner, who was off duty at the time, responded to an unfolding emergency at the southern end of Bulli Beach on Sunday afternoon where two girls were struggling to swim in a strong rip current about 100 metres from shore.

He swam out to rescue the girls, while lifesavers from Woonona and Bulli club and a support operations crew on a jetski from Thirroul responded to scene.

"Thankfully a quick thinking surfer heard the screams and paddled over to provide immediate assistance prior to me swimming out there," Mr Turner said.

"The three family members were able to keep afloat on the surf board while I reassured the young girls that rescue boats were arriving.

"The large family group from western Sydney were very grateful for the quick response from the surfer and myself and were overwhelmed how quickly things changed in the ocean.

"Two lives were definitely saved."

Mr Turner said he had to advise the family members to not enter the water to help as the current was strong.

He reminded the public to call Triple 0 if they saw a person struggling in the water, run to contact the nearest lifesaver and only enter the water, with a flotation device, if they were an experienced swimmer.

Mr Turner said the large family group told him they tried multiple beaches before finding car spots at Austinmer and they set up at the southern end of the beach because they were conscious of social distancing from other beachgoers who were positioned between the flags.

Mr Turner's advice was for people to spread out along the beach to maintain social distancing, then walk up to the flagged area and swim in the patrolled location.

"If we can't see you, we can't save you," he said.

"There was a lot of good luck and quick-thinking responses from surf lifesavers and boardriders.

"Time is critical if someone is caught in a rip, especially if they are unfamiliar on what to do.

"There were a lot of rescues for the region on Sunday, with most outside the flagged areas. There could have been drastic consequences."

Mr Turner added that two people were caught in a rip current at Puckeys Beach on Sunday before they were rescued, given first aid and transported to hospital.

Lifesavers also conducted multiple rescues for people who were injured or suffered medical episodes while in the water before paramedics assisted.

Volunteers also helped locate missing people, including a child at Corrimal, who were found safely.

Mr Turner said crews would be ready to respond to multiple calls for assistance on New Year's Day if required but patronage at the beach heavily depended on sunny weather.

He said lifesavers would be on jet skis patrolling unpatrolled locations and educating people about the need to swim between the flags.

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