The 21-year-old from Palmerston claimed the top spot at the first round of the motocross Women's World Championship. Racing on a boggy track in Pangkal Pinang, Indonesia, Duncan had to come from behind to edge in front.
The Yamaha rider finished fifth in race one on Saturday, after falling at the first corner and having to play catch-up.
The mud on track proved difficult to navigate, with defending champion Livia Lancelot and Kiara Fontanesi also falling victim.
The track was changed for Sunday, with the worst-
affected part removed. However, rain fell throughout the race, making for another boggy course which led to plenty of mistakes throughout. Duncan was among those affected, as she gave up an 11-second lead to Fontanesi.
However, she was able to bounce back and went passed the Yamaha rider to take the 25 points and finish second on the overall standings.
She was then upgraded to first overall when Nancy Van Der Ven, who won race one, was disqualified from race two due to outside assistance.
''It started off like a bad day, but we turned it into a good day so I couldn't be too much happier about how it went in the end,'' she said.
''It was crazy. The conditions were pretty brutal out there. It was even tough to make it round a lap of the track, we'd had that much rain in Indonesia.
''Riders were getting stuck all over the track, the bikes were pretty much halfway covered in mud. Pretty crazy conditions, but you just had to make the most of it.''
The standings see Duncan on 41 points, with fellow Yamaha rider Nicky van Wordragen snapping at her heels in second on 40 points.
Last season, Duncan's first in the series, she was similarly well-placed after the opening rounds.
However, at the German event she collided with a photographer who had come too close to the track. The resulting hand injury put an end to her season and she missed the final two rounds. .
She is now home for three weeks, before returning to her Belgium base and remaining in Europe for six months.
The next leg of the series is to be held in Italy on April 16.
