Surf Life Saving New Zealand club development officer for Otago and Southland Steph Laughton said some adults were dropping off youngsters at beaches and leaving them to their own devices.
In other cases, parents were at the beach but fell asleep or otherwise became unaware of what their children were doing.
Miss Laughton, also a volunteer patrol captain at the St Clair Surf Life Saving Club in Dunedin, warned those situations could be dangerous, despite the presence of lifeguards.
Lifeguards monitored swimming between the flags and maintained overall beach safety.
They were not there to be child-minders, in the absence of parents, and were not intended to be the first line of defence in water safety.
Also, sea conditions could change quickly.
"Waves can come in and sweep the little kids off their feet, and before you know it, they're out to sea."
She urged parents to know where their children were at all times and remain within touching distance.
Unsupervised children under the age of 16 had long been a safety issue at Otago beaches.
She had seen children as young as 8 unsupervised at the beach.
The swimming ability of youngsters varied widely and even good swimmers at indoor pools could find sizeable waves and strong currents at the beach a "shock".
St Kilda club surf lifeguard Ant Jackson said young people sometimes made their own way to beaches, unaccompanied by adults.
The need for proper parental supervision was emphasised in "beach education" classes conducted through Otago schools, he said.
"You want the parent to be watching out for the kids."
• Kaka Point, Warrington, Brighton, St Kilda and St Clair beaches are patrolled. As well as weekend volunteer patrols, which began on October 30, noon to 5pm, weekday patrols started last Monday and run from 11am to 7pm until January 28.
