His son Ethan, 17, and wife, Mandi, 45, heard the commotion and went outside.
Australian Deer Association's Barry Howlett told the Herald Sun it's now mating season for deer and many will become aggressive and territorial.
Mr McDonald died from his injuries at the scene while his wife was flown to the Alfred Hospital in Melbourne with life-threatening injuries to her upper body and leg.
Local resident Patrice Larkin said she knew the couple well.
The 46-year-old man entered the stag's enclosure in the morning at Moyhu in Victoria state, Police Sergeant Paul Pursell said.
ABC reported the stag had been kept at the property near the town of Wangaratta, 200km northeast of Melbourne, for six years.
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The animal was then killed "due to concerns for the paramedic's safety on scene", police said.
"It is unusual, but not unheard of, for captive deer to attack humans, particularly during the Autumn mating season", the association said in a statement.
A warning has been issued after a man was killed by a deer in Victoria.
"They are a wild animal and a wild animal that loses its fear of humans is a really risky thing".
He added that Australia's deer farming industry, where valuable velvet is extracted from antlers, has "declined greatly over the last 20 to 30 years". Howlett said fallow and red deer become particularly aggressive during the mating season. The Parks Department has proposed funding a bigger hunt to control the growing number of feral deer throughout Victoria.
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