"When paramedics arrived they found him lying on the side of the road. He had massive injuries and had to have advanced life support intervention," said a medical spokesman.
Mackay, an Australian Sevens international, was a replacement in the game at the ABSA Stadium. He was said to be in a stable condition at Durban's St Augustine's Hospital.
The Brumbies are remaining in South Africa for another week as they prepare for their clash with the Cheetahs in Bloemfontein at the weekend.
"Shawn is a popular member of the playing group and obviously our thoughts are with him. While we are still gathering details our primary concern is for Shawn's health and safety," said Brumbies spokesman Nick Smith.