Three siblings watched in horror as their parents drowned while swimming on a beach on the NSW north coast, police say.

The Sydney family was holidaying at South Ballina when the tragedy occurred about 7pm (AEDT) on Tuesday.

Police said the 44-year-old woman became caught in a rip when her 42-year-old husband went in to save her.

The couple's daughter used her mobile phone to call triple zero when she noticed her father also began to struggle.

Another daughter ran to get help from a nearby caravan park where they were staying.

A beachgoer recovered the man from the water and began performing CPR until paramedics arrived, Surf Life Saving NSW said.

The Ballina Surf Life Saving jetboat and lifesaver helicopter located the woman unconscious in the water.

Attempts to revive the couple were unsuccessful.

The girls, aged 14 and 17, and their nine-year-old brother were taken to Ballina Hospital and treated for shock.

They are now in the care of a relative, police said.

"This acts as a tragic reminder of the risks of swimming at unpatrolled beaches or after patrol hours," Surf Life Saving spokesman Dean Storey said.
The couple are yet to be formally identified.