South Africa made a bad start against the pace of Mitchell Starc and Pat Cummins before Agar’s triple strike sent them hurtling to their all-time lowest T20 total of 89.
It was also the host country’s biggest defeat in the format.
Man of the match Ashton Agar finished with career-best figures of five for 24.
“It was really exciting, a highlight of my cricketing life,” he said.
But he gave praise to the fast bowling attack of Mitchell Starc, Kane Richardson and Pat Cummins who started South Africa’s collapse.
Starc made a crucial breakthrough when he bowled South African captain and batting star Quinton de Kock for two with a fast, swinging delivery in the first over of the home side’s reply.
South Africa were reeling at 40 for four when Ashton Agar came on to bowl.
He had Faf du Plessis caught at deep cover with his fourth delivery, trapped Andile Phehlukwayo leg before next ball and Dale Steyn superbly caught at slip by captain Aaron Finch to complete the hat-trick.
Smith and Finch (42) set up Australia’s big total with an aggressive second wicket stand of 80 off 52 balls, benefitting from some loose South African bowling.
“I’m not quite sure what happened,” said De Kock.
“Their energy was there but the skills weren’t good enough.” By contrast Australia’s bowlers were on target immediately.
“The way we attacked the crease and bowled a really good, aggressive length was crucial,” said Finch.
“We pride ourselves on not giving away width with the new ball and making the batsmen hit good shots off good balls.”
It was a successful return to South Africa for Smith, who was stripped of the captaincy and banned for a year after the ‘Sandpaper-gate’ scandal in 2018.
David Warner, who was also banned for a year for his part in the affair, hit a four off the first ball of the match but was caught at fine leg off the next delivery from Steyn, ending a sequence in which he had scored 267 runs and only been dismissed once in his previous six T20 internationals.
The win extended Australia’s unbeaten run in the format to nine matches, with eight wins and one no-result. Their last defeat was in November 2018.
The second match of the series is at Port Elizabeth on Sunday with the third and final game in Cape Town on Wednesday. (AFP)