A two-year-old boy has died after being pulled unconscious from a backyard pool and suffering a cardiac arrest.

The boy was pronounced dead on Monday afternoon, a little less than 24 hours after an ambulance was called to a house in Cronulla, with reports of a child under water.

He was found by a 15-year-old girl, who performed CPR after pulling him from the water, Inspector Mark Walker told ABC radio.

"It appears he's gone in undetected and we're not exactly sure how long he's spent in the water either," he said.

"It's a tragic incident and it's a reminder to supervise children around pools at all times."

A later call stated that the child was seen to have gone into cardiac arrest.

The toddler was later taken to the Sydney Children's Hospital at Randwick.

A neighbour said a young family with three children lived at the house and were hosting a party.

She said the toddler did not live at the house. It is believed he was visiting with his mother and stepfather.

“I just saw the ambulance and people crying; there were young girls and children,” the neighbour said.

“It’s just very distressing. It’s a young family and these things shouldn’t happen.

“It is a small street, not that we’re very close, but we all look out for each other.”

Another neighbour, who lives next to the house, said she heard children playing in the backyard throughout Sunday afternoon.

In another pool incident on Sunday, a three-year-old girl was pulled unconscious from the water at the Sydney Olympic Park Aquatic Centre about 5pm.

Pool attendants performed CPR and the girl was taken to The Children’s Hospital at Westmead.

She was in a stable condition on Monday morning, a NSW Police spokesman said.