Sydney boy dies after being found in pond

A young Sydney family is mourning the loss of a three-year-old boy who died after being found in a neighbour's pond.

Bob Watt Jr had been riding his bike at his family's Ingleside home in the city's northern beaches on Monday afternoon when he ventured onto the adjoining property.

Neighbour Joe Gigliotti had been doing property maintenance work when the boy went missing.

"He was talking to me about 10 minutes before the accident and he had to show me his toy truck with a trailer and a car on top," Mr Gigliotti said on Facebook with a photo of Bob and his older sister.

"He was riding the bike up and down the driveway, and then we couldn't see him anymore and the father came and asked me, 'Where is Bob?'," Mr Gigliotti told The Daily Telegraph.

"I saw the father walking up and down the drive way and next door looking for him. And then I was up on the roof of my property doing some repair work and the next thing I hear is all these sirens," he said.

Robert Watt reportedly found his son's bike on the edge of the dam and then saw the little boy floating unconscious in the pond.

The three-year-old was taken to Sydney Children's Hospital in Randwick in a critical condition but died overnight.

Mr Gigliotti paid tribute to the "gorgeous little boy" on Facebook.

"I feel so sad for his loss. My heart goes out to his mum and dad, Robert & Janet Watts with the other children for this terrible tragedy," he said.