The Australian, best known as a Team Sky rider who played a key role in supporting Sir Bradley Wiggins to the 2012 Tour de France title before helping Chris Froome accomplish the same feat in 2013 and 2015, is competing in his first year for BMC Racing.

All the major contenders, including overall leader Simon Gerrans, began Willunga Hill together with just three kilometres to go in the 151.5km stage of a race which is the opening event in this year's International Cycling Union WorldTour. 

After plenty of flurries, it was the home rider who made the decisive move, accelerating away impressively to win by six seconds in 3 hours 34min 16sec.

“To be honest I never expected that one,” said the 30-year-old.

“Today the BMC guys were absolutely incredible and they backed me here, which is nice and I have only just started in this team. 

“It’s always nice to win in Australia and on top of Willunga for the third time in a row, I am really happy."

Colombia's Team Sky rider Sergio Luis Henao finished in second place on the day, with Canada's Michael Woods, riding for Cannondale, finishing third a further three seconds back.

Porte’s now lies second overall, meanwhile, nine seconds behind compatriot Simon Gerrans after receiving a 10 second winners' bonus.

Orica-GreenEdge's Gerrans hung-on to finish eighth today, 17 seconds behind.

He will almost certainly now secure a record fourth Tour title in tomorrow's flat final stage.

Henao moved into third overall, overhauling Australia's Jay McCarthy after the Saxo-Tinkoff rider proved unable to stay in contact in the closing stages.

Tomorrow's finale is due to consist of a 90km circuit race around Adelaide.