Close behind them, the race to stay in medal contention was being played out between O'Grady, Mark Leishman (Rotorua) and Jan Francke (Czech Republic).
Half way through the 26km ride it was Thorne who put the hammer down, opening up a gap over his transtasman rival, and it paid off, with Thorne leading heading onto the run by a minute.
However, a calf injury that nearly ruled Thorne out of the race entirely, took its toll and before halfway round the 11km course Allen had the lead back and was putting time into all but one of his competitors.
It was O'Grady, who, after a tough mountainbike, ran himself from fourth to second place. Thorne held on for third.
"I was running scared, I knew Graham was a great runner, I had to push it hard the entire race," Allen said.
"This is a special race to win, I was here last year and got fourth but I loved it, it is one of the best races in the world. I had to come back with Jacqui and I'm so pleased I did, we couldn't have asked for a better result."
Slack had things more her own way in the women's race, leading the field from start to finish and taking the title by 5 minutes.
"I didn't know the girls I was racing against very well, so I just had to go as fast as I could for the whole race. Fortunately it was enough to hold on for the win."
