Porte waited until the final two kilometres of the Paracombe climb to make his move and quickly distanced the peloton and the two riders who eventually joined him on the post-race podium.
He finished the 148.5km stage from Stirling to Paracombe 16 seconds ahead of Gorka Izaguirre (Movistar) and Esteban Chaves (Orica-Scott).
"I had nightmares of two years ago when we played cat and mouse there and it didn't work," Porte said. "It's nice to win on the Paracombe. It's good to get a win this season and now we will try and defend this jersey."
The overnight cool change was a welcome respite for the field, with temperatures in the mid-20s after Tuesday's Barossa furnace.
Jasha Sutterlin (Movistar) was the first major mover on the stage with a solo break which lasted to the 35km to go mark after a combined chase in turns by Orica-Scott, BMC and Sky pulled him back.
A thinning peloton was all together at the base of the climb and stayed that way until Porte made his decisive move after testing the resolve of his rivals with a momentary feint.
Izagirre struggled to contain Porte as Chaves, momentarily left behind, recovered to join what turned out to be a futile chase.
While Porte won the stage by 16 seconds, the stage winning time bonus gives him a 20 second lead on Izagirre with Chaves a further two seconds back.
“Richie made three accelerations very fast, but I could not stay with him," Chaves said. "I am happy for the third, and we will continue to try for the win.”
“You never know what will happen and we will keep fighting like always. Willunga is like seven minutes (of climbing) but today was harder because it was climbing all day. I am excited (about the coming days) so we will see.”
Porte's position now appears unassailable, and as a three-time winner in the critical second-last stage at Willunga is on track to win his first Santos Tour Down Under.
"To be able to ride away like that gives me good confidence," Porte said. "But there are some stressful days coming up. We won't be counting our chickens until they hatch."
The Tour is Porte's first race since crashing out five months ago in the Rio Olympics road race.
The Adelaide race is an important early-season objective for Porte, who is aiming this year for the Tour de France podium.
Porte cautioned last week that he was unsure of his form, but there were no doubts now.