Adelaide - Richie Porte lost his unofficial King of Willunga crown but won his second Tour Down Under title in a punishing hill climb finish to the final stage of the UCI season-opening event in Australia on Sunday.

Porte, the 2017 winner, went into the final stage trailing defending champion and race leader Daryl Impey by two seconds but the South African Mitchelton-Scott lead rider couldn't go the pace on the final climb and lost more than 30 seconds to finish sixth overall.

The Australian from team Trek Segafredo unleashed his trademark surge up the steep three-kilometre climb in a bid for a seventh win in the demanding Willunga Hill stage, but was overtaken by England's Matthew Holmes in the final stretch.

"I asked a lot of my team all week and they did not miss a beat. Thanks Trek-Segafredo, what a team," Porte said.

"It was a little bit panic stations there in the middle of the stage (behind the breakaway).

"We had to do the ride for the whole day. My team today were absolutely fantastic. It's been an incredible week."

Holmes, who was part of a 26-strong breakaway group which maintained a gap of up to four minutes on the main peloton for much of the stage, stuck to Porte's back wheel as the Australian launched his attack the second time up Willunga Hill.

The Lotto Soudal rider swept past Porte through the final corner to take his first professional win.

"I've never really raced up a climb," Holmes said. "This is my second hilltop finish this week, and it seems to suit me."

Impey, who was chasing three straight TDU victories, was a beaten man as he laboured towards the finish line.

"I came over the line spent. I knew I was out of it there once I got dropped and I just kept fighting to the line, trying to limit the damage and trying to save the podium," Impey said.

"I wasn't on a great day today. I struggled a bit there on the bottom slopes, perhaps paying for quite a busy week but, that being said, we gave it our all."

Italian Diego Ulissi (UAE Team Emirates) finished second overall, 25 seconds behind Porte, with Germany's Simon Geschke (CCC Team) third.

Australian individual world time trial champion Rohan Dennis, riding for Team Ineos, finished fourth overall and said he had work to do ahead of the international season.

"I felt good for January, but I've got a bit of work to do before July and my big goal of the Tokyo Olympics and for the Giro I have to get better at climbing again," he said.

"I did alright today, but it's only a 3km climb, not 10-15km like the final week of the Giro d'Italia."

1. Matthew Holmes (GBR/Lotto Soudal) 3hr 24min 54sec

4. Bruno Armirail (FRA/Groupama-FDJ) +7

7. Simon Geschke (GER/CCC Team) s.t.

10. Simon Yates (GBR/Mitchelton-Scott) +23

2. Diego Ulissi (ITA/UAE Team Emirates) +25 sec

5. Dylan van Baarle (NED/Team Ineos) s.t.

8. George Bennett (NZL/Team Jumbo Visma) +46