A heroic Victorian man has drowned while trying to rescue a child caught in a rip at a beach in Wollongong, south of Sydney, police said.

Police were called after reports four children were struggling in the surf at Wollongong City beach about 4:30pm on Sunday.

Acting Inspector Cheryl Dunlop said the children's father got the two youngest to shore while an off-duty paramedic helped a 10-year-old boy to safety.

The good Samaritan, Shaun Oliver, swam out to help a 12-year-old boy, but the boy was swept further out to sea.

Shaun Oliver died after attempting to help a boy struggling in the surf at Wollongong City Beach ( Supplied: Facebook )

"A 32-year-old man also entered the water to help rescue the children, but he got into trouble himself," Inspector Dunlop said.

A surfer, who was at the beach, then helped the 12-year-old boy to shore.

Mr Oliver was found a short time later face down in the water.

Three police officers arrived and brought him to shore and paramedics gave him CPR, but he died later in Wollongong hospital.

Two of the four children and the off-duty paramedic were taken to hospital for assessment.

Tributes to 'hero' dad

Friends and family of Mr Oliver paid tribute to him on social media.

On the crowd funding page GoFundMe, his family said Mr Oliver and his wife Carla were childhood sweethearts.

"Sadly not long before their tenth wedding anniversary, Shaun was faced with an ultimatum.

"He chose to protect those who could not protect themselves.

"And he, and his family have paid a heavy price."

The Keysborough Football and Netball Club, where Mr Oliver played as a junior and senior, tweeted that news of his death was "devastating".

On Facebook, Daniel Jackson said: "You went out selfless trying to help. You are a good man Shaun".

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Remarkable acts of bravery

Detective Inspector Brad Ainsworth said all the people who went to help the children acted heroically.

"There was just remarkable acts of bravery all around," he said.

Detective Inspector Ainsworth said the 32-year-old man who died paid the ultimate sacrifice for coming to the aid of others.

"He was here on business, I believe, from Victoria. He's a married man with three children.

"I believe he was just walking along the beach and he's just acted."

But he said the incident was avoidable.

"When you say, 'would they [the swimmers] know about the surf?' I would say, 'no'.

"Yesterday there was treacherous conditions, there was a heavy surf, the beach was closed, it wasn't patrolled.

"Unfortunately, they've entered the water and it's resulted in the death of a hero."

Surf Life Saving NSW Illawarra duty officer Daren Weidner said swimmers needed to heed warnings about dangerous surf conditions.

"They've ventured out into dangerous seas. There's a rather hazardous swell warning issued this weekend. We've seen seas and waves of three to four metres," he said on Sunday afternoon.

"People need to heed the warnings and make sure they don't go in dangerous surf and unpatrolled beaches."

Inquiries into the incident are continuing and police will prepare a report for the coroner.