Porte attacked with 2km to go leaving Sergio Henao (Sky) and Esteban Chaves (Orica-Scott) in his wake. From there is was a solo run to the finish and a fourth consecutive victory on the now legendary South Australian climb.
Nathan Haas (Dimension Data) was next across the line after he recovered from the pace applied while sitting in the lead group while Chaves also rebounded to finish third after the 151.5km stage from Mclaren Vale to Willunga.
With one stage to go the final general classification is now set with Porte leading Chaves by 48 seconds while Haas claimed a surprising but well-earned third at 51 seconds.
"That hurt more than the other three I've gone up here," said Porte. "I went from quite a long way out. It was just incredible teamwork today and I'm just happy I could finish it off for the guys.
"This is not an easy climb so to win it is great for morale. Obviously for me the biggest goal this year is July at the Tour de France so things are going well."
Earlier in the stage a five-man break containing Thomas De Gendt (Lotto-Soudal), Will Clarke (Cannondale-Drapac), Jeremy Maison (FDJ) and Jack Bauer (Quick-Step Floors) formed but were kept on a tight leash by the peloton with the time gap hovering around the two to three-minute mark for the majority of its time out front.
Clarke was the first to falter as De Gendt kept the pressure on in his quest to build on his mountains classification lead on the first ascent of Willunga, while from behind, a threatening chase came from Australian Chris Hamilton (Sunweb-Giant) and Giovanni Visconti (Bahrain-Merida) with 25km left to go.
With 5km to go the race was on as the break and chase succumbed to the pressure led by Orica-Scott and Quick-Step floors while Porte comfortably managed the increase in tempo by sitting on Chaves's wheel.
The final 90km stage takes place on the streets of Adelaide on a circuit where Porte will be wary of any dangers to his position
"Tomorrow's not a procession," Porte said. "I'm in a good position and I think I can stay out of trouble. I just want to finish it off tomorrow.
"I've always wanted to win this race since I raced it with UniSA. I've said it before, I'd love to have my name up there."