MacDonald was understood to have "flatlined'' twice after he fell to his knees and had a seizure minutes before half time during Saturday's game.
Pukekohe Volunteer Fire Brigade's senior station officer Graeme Wilson said MacDonald was talking to the team doctor when he had a "fit'' and fell unconscious.
His condition deteriorated rapidly and he went into cardiac arrest just minutes after being taken from the field on a stretcher.
"It was pretty scary and definitely life-threatening for a while,'' Mr Wilson said.
"I could tell by the anxiety of the ambulance officers.''
Ambulance officers used a defibrillator on MacDonald after CPR and a shot of adrenaline failed to bring back his pulse.
Yesterday, Southland head coach David Henderson said: "As coaches, we knew he was in some serious trouble before half time.''
They had tried to shield the team to stay focused on the game so the medical team could do their work and play could continue.