Mackay suffered spinal injuries and a broken leg as well as multiple facial fractures and lacerations after being hit by an armoured response vehicle while crossing a road.
The accident occurred at about 4am in Durban when Mackay and a number of other Brumbies players were returning to the team hotel following a night out after their Super 14 loss to the Sharks on Saturday night.
A Brumbies spokesman said police were investigating the incident but it is believed Mackay was not "skylarking" at the time.
Mackay, 26, suffered a cervical spinal fracture and dislocation in the accident.
He was in a critical but stable condition and was expected to remain in an induced coma as he undergoes further medical treatment.
Brumbies team doctor Warren McDonald, who was working with staff at Durban's St Augustine's Hospital, said Mackay was receiving around the clock intensive care and treatment.
"Shawn has sustained severe head and leg injuries but has received first-class medical treatment from the time of the accident," McDonald said.
"He is currently in a critical condition but is stable and we won't know the full extent of his injuries until he is brought out of a coma in two days time." Brumbies management has been in contact with Mackay's family and partner and are arranging to fly them to Durban as soon as possible.
It is believed his parents will depart for South Africa on Monday to be with their son.
"We would request that people understand the sensitivity of the situation and ask that people please respect his and his family's privacy at this time," McDonald said.
Brumbies media spokesman Nick Smith said the players were obviously in shock.
The team was set to go on safari on Monday but have cancelled those plans and will stay in Durban.
Mackay, who came off the bench in the 35-14 loss to the Sharks, joined the Brumbies at the start of the season from the NSW Waratahs.
He has been a member of the Australian Sevens side since 2004 and also spent five years playing rugby league with the Sydney Roosters' junior side.
The Brumbies are scheduled to travel to Bloemfontein on Thursday ahead of this weekend's match with the Cheetahs.
