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Northey trailed leader American Carter Jones (Bissell Pro Cycling) after this morning's individual time trial in Winton but the tour's defining moves were made in the final stage.
Led by Calder Stewart's Hayden Roulston and Michael Vink, a break was made early and Northey and his Node 4-Subaru teammates made sure it was one they weren't going to miss.
"It was dangerous but, in hindsight, it worked well for us," Northey said. "I knew it was going to be the winning move and my teammate managed to jump me across to the break and that was it, we were away from there. It was pretty awesome and a good feeling to come in knowing I'd taken the title. I still can't believe I've won.
"This is the biggest tour I've won. It's my eighth time here and I never thought I would win it."
Northey, 25, climbed his way into the yellow jersey early in the tour, winning Tuesday's stage finish at Bluff Hill and maintaining his hold on the lead throughout the two stages on Wednesday. Jones took the lead after Thursday's stage to the top of the Crown Range and maintained a slender four-second lead going into the final day.
He increased it to 17 seconds after this morning's time trial but finished more than three minutes behind the leaders on the final stage to drop to fifth overall.
Today's final stage, raced in cold and wet weather, was reduced from 87km to 79km but there was more than enough road for the breakaway of five riders. Roulston took the stage win from Vink, and the pair rocketed up the overall standings to finish the tour second and third respectively.
Vink had earlier won the morning's time trial in Winton and said, while that had not earned him much in terms of time, it had added to his confidence.
"We weren't taking anything into the last day, but today we pretty much took it all, apart from the yellow jersey," Vink said.
"Today's stage was about riding hard and sorting out the contenders from the pretenders - and that is exactly what we did. It was only 80km but we certainly blew the field apart after a hard week of racing."
Joe Chapman (Creation Signs-L&M Group Racing) won the King of the Mountains, Sam Witmitz (Share the Road) claimed the Sprint Ace title and Northey's Node 4-Subaru team also won the team's classification.