Authorities say the victim was standing at the back of his truck going through the mail on Prairie Drive in North Babylon around 2 p.m. when the Lincoln Continental veered off the road and struck him.
"I was in my house, and I heard a boom," neighbor Jason Lesley said. "Then, about three or four seconds later, another boom, then what sounded like a shot of water coming up. And I was realizing the hydrant being under the gentleman's car."
PHOTOS: Scene of crash where postal worker was struck
The victim suffered extensive injuries to his legs and was airlifted to Stony Brook University Hospital.
"I came outside and I saw a car smashed into my telephone pole," said Lesley, a registered nurse at Jamaica Hospital. "I saw my mailman about 30 feet down the block laying on his side. I thought he was deceased. He wasn't moving...I walked up to him, and his leg was severed. It was completely off."
The driver of the Lincoln also struck a fire hydrant and a telephone pole. He or she refused medical attention at the scene.
The Lincoln's driver will be charged with reckless driving.