He was 26.

Mackay was hit by a car at 4am on March 29.

He had spine and leg injuries and was put into a medically induced coma by South African doctors.

They brought him out of the coma late last week and displayed initial signs of progress when he responded to questions by blinking.

However, a bacterial infection had hindered his recovery and his promising condition deteriorated rapidly.

Mackay had a heart attack this morning and did not recover.

Brumbies chief executive Andrew Fagan said the entire Super 14 franchise was devastated.

"Shawn made an enormous contribution to Australian rugby through the Australian sevens and then the Brumbies," Fagan said.

"But most of all he was a great man and an outstanding character.

"He will be dearly missed by all of us. This is a tragedy.

"Our thoughts and prayers are with his family at this difficult time."

Mackay had been admitted to St Augustine's hospital in Durban with a cervical spinal fracture and dislocation, a fractured skull as well as a broken leg and multiple facial fractures after being struck by the vehicle.

The incident occurred about 4.15am Durban time when Mackay and a number of other Brumbies were leaving a nightclub, having wound down after their Super 14 loss to the Sharks the previous evening.

Mackay's parents, John and Leonie, and girlfriend Trish flew to South Africa on Tuesday to be by his side.

The Brumbies were flying back to Australia on Monday after completing their three-match trip to South Africa with a win over the Cheetahs in Bloemfontein at the weekend.

From Sydney club Randwick, Mackay was a hard-running 104kg, 195cm backrower or lock who also had the pace and skill to be a regular for Australia on the international Sevens circuit for several years.

He made his Super 14 rugby debut with the NSW Waratahs in 2006, against the Brumbies, and played six games for his home state before switching to the Brumbies this season.

He played in two games this Super 14 season, both on the South African tour, coming off the bench against the Lions in Johannesburg and against the Sharks the night before he was struck by the car.

Full coverage in tomorrow's Canberra Times