That gave the Australian cycling star the impetus to launch his trademark solo attack and claim the Tour Down Under's Queen stage for the sixth-straight year.

The King of Willunga was comfortable settling for a fourth runner-up result overall at the Tour as South African Daryl Impey made race history.

Impey became the first rider in 21 Tours to win back-to-back titles when he finished third at Willunga, with the same time as Porte.

As expected, the stage result and the Tour came down to the final 3km climb to the Willunga summit finish.

Porte attacked with 1.3km left, shaking off Dutch rival Wout Poels (Team Sky) and soloing across the line.

But the Tasmanian later revealed that early on the climb, he was unsure about his prospects after Sky had tried to shake up the end of the stage.

"I thought I'd had it there but, when I heard Kim Andersen (his team sports director) telling me on the radio there that everyone behind me was suffering too, it was a second wind," Porte said.

"But the last 300m was probably the longest 300m. It really hurt. It was a sweet victory."

It is Porte's first win at Trek-Segafredo and he will head to Victoria for next Sunday's Cadel Evans Great Ocean Road race and the Herald Sun Tour before heading to Europe.

Porte noted after Sunday's final stage that unless the week-long Tour had a second summit finish, he would struggle to repeat his 2017 overall win.

"It is just nice to get a victory ... it is always good to start with a new team with a win," he said.

Impey dedicated the overall title to Australian teammate Mat Hayman, whose 20-year professional career ended with Sunday's stage.

The title was a triumph for the Australian Mitchelton-Scott team.

Another Australian teammate, rising star Lucas Hamilton, paced Impey perfectly up the second of the two Willunga climbs in the 151.5km stage from McLaren Vale, south of Adelaide.

Porte beat Poels for the stage and Impey was right behind them.

Impey beat Porte by 13 seconds overall and Poels was third at 17 seconds.

New Zealander Patrick Bevin (CCC Team) started the stage as race leader, seven seconds ahead of Impey, but predictably dropped off the pace on the first Willunga climb and finished five minutes off the pace.

Bevin clearly was suffering from his injuries after he crashed with 10km left in Saturday's stage.

While it was a bitter blow to lose any chance of becoming the first NZ rider to win the Tour, Bevin gained many fans for first gaining the race lead and refusing to give in to his injuries.

Australian Associated Press