Instead of being sucked into a speed duel, Du Plessis eased Petty Cury back to a comfortable trail, which proved the winning of the race.
Just as Petty Cury found a home-straight gap, the second favourite, Durham Town, loomed on a long, sweeping run from the back of the field.
Durham Town looked certain to win at the 220m, but the weight differential (53kg Petty Cury v 57kg) started to tell and the pair drew level then fought strongly to the finish.
In one of the closest non-deadheat finishes judge John Craig has seen, Petty Cury got the decision by barely a pixel.
Durham Town was having his final start for the Dean and Donna Logan stable before relocating to Chris Waller's Sydney operation.
The big chestnut leaves New Zealand with a hugely gallant effort. Colgan had to set him alight well before the home bend and the pair were six and seven deep around the bend.
He never stopped trying and will find Sydney conditions to his liking with lighter weights because of the greater array of sprinters.
Mark down Tyne Cot as a coming top liner.
He wasn't entitled to win on Saturday. Big, gangly, inexperienced types don't win at their first experience right-handed, particularly at Ellerslie.
That's unless they're as good as Tyne Cot.
The victory surprised rider Noel Harris. "At the 200m I thought we were a chance to run a nice third," said Harris.
"Then at the 50m I was saying: 'Yes, we can win, no we can't' and he got there.
"He's that good he's a bit scary."
