A THREE-YEAR-OLD boy was bitten by a 2½-metre brown snake while playing in a river north-west of Sydney yesterday, the state's second serious snake bite incident in three days.

The boy was playing in the shallows of the Colo River, north of Kurrajong, at about 3.15pm when he was bitten on the arm by the fully-grown brown - Australia's deadliest snake.

A number of bystanders who witnessed the incident were trained in first aid and immediately called triple 0 before applying a compression bandage to the boy's arm to prevent further circulation of the snake's venom.

After receiving treatment from paramedics from the NSW Ambulance Service, the boy was airlifted to the Children's Hospital at Westmead at about 4.30pm by helicopter.

A hospital spokeswoman said the boy was conscious and breathing when he arrived and was in a stable condition last night.

A spokeswoman from the NSW Ambulance Service said the bystanders had provided "perfect first aid".

"The compression bandage was exactly the right thing to do," the spokeswoman said. "It's not clear exactly what took place but we believe he was playing in the river at the time."

A young boy was bitten at St Marys on Saturday afternoon after attempting to pick up a black snake which he thought was a stick.

The boy was taken to Mt Druitt Hospital and was released on Sunday night having recovered.

Known for their extremely potent venom, brown snakes cause more snakebite deaths in Australia than any other.