The family of a young man killed in an alleged hit-and-run have described the driver as a “coward”, saying their “heart and soul” has been ripped away as they continue searching for answers.

Tamatea “Tama” Harrison, 20, died after he was struck by a car on Middle Rd at Greenbank, south of Brisbane, about 8.45pm on Monday.

WATCH THE VIDEO ABOVE: Family mourns man killed in Green Bank hit-and-run

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Police allege the driver fled the scene, leaving Harrison critically injured.

His brother, who witnessed the incident, and friends desperately tried to save him before paramedics arrived, but Harrison died at the scene.

On Wednesday, flowers, photographs and fishing gear now mark the roadside where his life ended.

To his grandmother Cheryl Tomlinson, Harrison was “her warrior”.

Standing beside the growing memorial, Tomlinson said the loss still felt impossible to accept.

“Even now, my whole body is stiff, because I want Tama to walk back through that gate,” she told 7NEWS.

“And I know that he’ll never do that again.”

Tama Harrison’s grandmother Cheryl says the family has been left searching for answers. Credit: 7NEWS

Messages left at the roadside memorial speak to the heartbreak of those who loved Tama Harrison. Credit: 7NEWS

Heartfelt tributes continue to be left where Tama Harrison lost his life. Credit: 7NEWS

The family’s heartbreak has been compounded by the circumstances surrounding Harrison’s death.

Tomlinson said his parents, Natalie and Jamie, rushed to the scene after learning what had happened.

“All Natalie could say, ‘I couldn’t even hold my son,’” she recalled.

“I couldn’t even say my last goodbyes to him.”

Tomlinson said knowing the driver allegedly left the scene while Harrison was critically injured has made their grief even harder to bear.

“That’s a coward act, and they need to face up,” she said.

Tama Harrison had stepped up to support his family long before his life was tragically cut short. Credit: 7NEWS

Police and paramedics responded after the 20-year-old was critically injured in the alleged hit-and-run. Credit: 7NEWS

Detectives are investigating whether hooning played a role in the fatal incident.

A GoFundMe launched by Harrison’s aunt Anita and family friend Jenniffer Ross on behalf of his mother says the family has been left devastated by the sudden loss.

Describing the 20-year-old as the “backbone of the household”, the organisers said he stepped up to run the family business and support his parents after his father suffered a devastating injury.

“Any help, no matter the size, is deeply appreciated as we try to honour him and hold each other up through this unimaginable time,” they wrote.

A black Ford Falcon has been seized and is undergoing forensic examination. Police allege the vehicle was unregistered.

Investigators are also trying to identify the occupants of a white Toyota LandCruiser and a dark-coloured ute seen in the area at the time.

No arrests have been made yet.

Police seized a black Ford Falcon as part of their investigation into the fatal Greenbank hit-and-run. Credit: Queensland Police

Tomlinson fears her family may never learn exactly what happened that night.

“Why did this happen to us?” she asked.

“Everything is going to be so hard, but we just have to keep moving forward.”

Anyone with dashcam, CCTV or mobile phone footage from the area has been urged to contact police.