A 23-month-old boy has died two days after his twin sister, and nine days after they were both found unconscious in a backyard pool in Sydney's north-west.

Robbie and Charli Manago were taken to the hospital in an ambulance escorted by police on December 20.

A spokeswoman from Westmead Children's Hospital confirmed that Robbie died on Thursday.

Police said the Manago children were out of their parents' sight for only a short time before they were found in a Kellyville Ridge backyard, and the pool met all the necessary safety requirements.

"Obviously at this time of year we urge people to very vigilant around swimming pools, maintain the security and keep an eye on the children," Chief Inspector Gary Sims said on the day of the accident.

"Whatever happens in seconds is a life-changing situation for the families, and you can't imagine the grief the family are going through."

NSW drowning death toll at nine

Robbie's death takes the number of drowning fatalities in NSW to nine over the Christmas period, with a still unidentified man found yesterday afternoon at Belongil Beach near Byron Bay.

There are two more people missing in the water, with the search for a boy at Maroubra beach and a man in the Murrumbidgee River near Wagga Wagga continuing.

A report is being prepared for the coroner.