The 31-year-old BMC leader finished the 151.5 kilometre stage from McLaren Vale to the summit of Willunga Hill 20 seconds ahead of compatriot Nathan Haas with Colombian climber Esteban Chavas in third.
Porte leads Orica-Scott's Chavas by 48 seconds ahead of the 90-kilometre final stage around the streets of Adelaide on Sunday.
A group of four riders had broken away early in the race, but were caught well before the final 3.5 kilometre climb to the finish, with Porte launching his solo attack to clinch the stage for the fourth successive year.
"I tell you what, that hurt more than the other three I've won up here," Porte told reporters. "It was incredible teamwork today.
"This is not an easy climb - to win it is great for morale."
The six-stage race around South Australia is the first event on the 2017 UCI WorldTour calendar.
(Reporting by Greg Stutchbury in Wellington; Editing by Sudipto Ganguly)
