Sydney family grieves after four-year-old boy dies in ute submerged in Hawkesbury River

A Sydney family has been left devastated after a four-year-old boy died trapped inside a car submerged in the Hawkesbury River.

Bentley Hilton, from Cranebrook in western Sydney, was buckled up in the backseat of a ute his grandfather had reversed down a boat ramp at Lower Portland about 10.45am yesterday.

The man hopped out to unstrap a tinnie in the back when the ute slid into the fast-moving water.

It was low tide and there was a sharp drop-off at the ramp. The car was swept away, with Bentley still inside.

His body was found still in the submerged car hours later.

Bentley's mother Lauren said the thoughts of her son's final moments would haunt her. 

"I tried to close my eyes last night and all I seen was him get up and wanting to get out and yelling out for help, that's all I could see," she told reporters today.

NSW Police Inspector Ian Woodward said the child's family were devastated.

"They came here for a nice family day out and unfortunately it has ended in tragedy," he said.

The boy's grandfather has provided a statement to police.

Police are continuing inquiries and a report will be prepared for the coroner.