The Saints scrum-half came off the bench to round things off for the Springboks, who claimed a bonus-point 35-17 win in the Rugby Championship opener in Johannesburg.
Reinach had not played a Test for South Africa since 2015, but he showed no signs of nerves after being handed some game time during the second half.
The 29-year-old was players' and supporters' player of the year at Saints last season, having scored 17 tries in just 30 appearances.
Reinach was joint top scorer in the Gallagher Premiership, alongside Sale Sharks wing Denny Solomona, and was named in the league's dream team.
He will now be desperate to win a place in the South Africa squad for the World Cup, which begins in Japan in September.
But Reinach faces stern competition from the likes of Faf de Klerk and Herschel Jantjies, who started and scored twice against Australia on Saturday.