SA's Police Commissioner penned a letter to the state's 101st road casualty - his youngest son.

The whole of the South Australian police force is in mourning this week, following a tragic loss among one of their own.

South Australia's Police Commissioner Grant Stevens penned a letter today to honour the 101st person to die on the road this year – his youngest son Charlie.

Charlie Stevens was the victim of an alleged hit and run, on Friday night in Goolwa, approximately 90km south-east of Adelaide. He was just 18, and had been celebrating Schoolies at Goolwa Beach, when he was allegedly struck by a car being driven by another 18-year-old man. 

On Saturday night, Charlie passed away in hospital due to his injuries. He was surrounded by friends and family.

Watch: Charges upgraded after SA Police Commissioner's son dies from alleged hit-run. Post continues below.

South Australia's Deputy Commissioner Linda Williams broke down in tears as she confirmed Charlie's identity to the media, alongside the premier.

"As you can imagine, this is a very difficult statement," she said. "We always talk about this happening to other people but the reality is it can happen to anyone, any family or anyone in the community. The sense of grief is always enormous."

Today, South Australia's Police Commissioner Grant Stevens and his wife Emma penned an open letter, emphasising that their son Charlie wasn't just a number or a devastating statistic. 

In the two-page-long message, they referred to Charlie being the 101st life lost in South Australia this year.

"101 is Charles Hinchliffe Stevens – Chare, Charle Boy, Chas, Links, Steve.

"I am writing this sitting in a bedroom with dirty clothes on the floor, an unmade bed, six drinking glasses lined up on the bedside table, an empty KFC box next to the glasses, wardrobe doors left open and a row of skateboards leaning on the wall – it is a mess and it is perfect. This is where 101 lived," Stevens said.

"Cheeky, intense and funny – a lovable ratbag from the moment he could talk. He was frustrating as hell, but he was also the kid who would look after others, befriend the lonely and help those who were struggling. Intensity shone through as 101 committed to each new passion – Lego, BBL, scooters, footy, cricket, basketball, surfing, downhilling, Fortnight and his skateboard – it was all or nothing and it was always all."

Stevens ended the letter by writing to his son Charlie: "You lived life and gave so much to so many. You were a force of nature and we will never forget your beautiful, cheeky, disarming smile. Son, brother, grandson, uncle, nephew, cousin, friend, workmate, team mate. So much more than just a number on a tragic tally."

The Stevens family say they wish to raise awareness on the importance of organ donation and ask the community to talk about organ and tissue donation with family and friends.

Charlie's brother also shared a statement of his own previously, writing: "My best mate, my biggest rival, my number one fan, my little brother Charlie. It breaks my heart my days of being a big brother have come to an end, forever in our hearts, forever young and beautiful."

The man who allegedly killed Charlie has been released on bail.

The man, 18, faces four charges including causing death by dangerous driving, aggravated driving without due care, leaving the scene of a crash after causing death and failing to truly answer questions. 

He is due to face Adelaide magistrates court in March.

South Australia's Premier Peter Malinauskas said the thoughts of all were with the Stevens family.

"Grant Stevens has served South Australia as a member of South Australian police for over 40 years. His whole life he has dedicated himself to protecting others, protecting South Australians from evil, protecting us from disease," Premier Malinauskas said. 

"He has shown extraordinary leadership in some of the most difficult circumstances this state has ever confronted and it is just so unjust that he and his family have now had to endure this great tragedy themselves."

It takes just one minute to register as an organ and tissue donor. Donate a minute to give someone a lifetime, and register at donatelife.gov.au or with three taps in your Medicare app.

Feature Image: Supplied.