Bike crashes leave two fighting for life

A cyclist and a nine-year-old child are fighting for survival in Sydney hospitals after two separate bike crashes.

The nine-year-old boy was riding his bike on the road at Koonawarra in suburban Wollongong on Sunday night when he was hit by a small truck.

Several bystanders rushed to his aid while the truck driver allegedly left his details at the scene and drove off.

There have been unconfirmed reports the truck driver left because he had a distressed young child in his vehicle.

The injured child was airlifted to hospital with critical head and internal injuries.

Sydney Children's Hospital could not provide an update on the boy's condition on Monday.

Police later arrested the 35-year-old truck driver but he was released without charge.

Friends left messages of support for the young boy - identified as Corey - on social media.

"I no (sic) young Corey and he is a fighter and he will fight hard," Jodo Leigh Ward wrote.

"Thinking of you Corey," Donna George Siddle posted.

Barely 20 hours later, a cyclist was critically injured in a crash in Neutral Bay in Sydney's north.

The 59-year-old male bike rider and a Honda sedan collided just after midday on Monday.

The cyclist is in Royal North Shore Hospital in a critical condition.