Stage five of the MEPT-Crocodile Trophy was a 125km marathon with 1100 metres of elevation surrounded by dense Outback vegetation
Dekker attacked race leader Leandre Bouchard (BH SR Suntour KMC) and second placed Andrew L'Esperance (Forward Racing-Norco) to take the win at the Coffee-Skybury Coffee plantation finish.
“It was hard. A hard day. I attacked in the beginning and the two [Bouchard and L'Esperance] came with me," Dekker said.
"They were looking at each other as I attacked again and then they tried to get away, but after ten kilometres they sort of fell back to me and we worked really well together. You know, looked out for each other.
“I knew that they had to sort of really worry about their overall standing because they are so close in first and second, so I thought, maybe today is the day and I went for it and it worked out."
Dekker finished in 4hr 30min with a gap of 20 seconds ahead of L'Esperance and Bouchard who are locked in a battle for supremacy with only 1min 31sec separating the pair.
Bouchard leads L'Esperance while Dekker not only leads his veteran classification but is third in the overall classification, ahead of Australian Ondrej Slezak (Quantum Racing Team) by 40 minutes.
“You know, in the general classification I'm so far behind and in my own master category quite far ahead, so this was a real goal for me and I'm really happy about the big stage win boomerang trophy," Dekker said.
The Crocodile Trophy was only his second ever mountain bike stage race for Dekker, who turned to the discipline to keep his fitness levels high. His inspiration for entering was the 2016 performance of Dutch compatriot and new world time trial champion Annemiek van Vleuten.
"I love this race. I can't see why I couldn't do this again, we'll see, but this is a historic race. I'm here because I realised that I love cycling. I thought I'd never do the Crocodile Trophy or any more stage races at that, but 11 months ago when the race was on I was following Annemiek a bit and I thought, you only live once."
Women's race leader Haley Smith (Norco Factory Team) finished as 15th overall and is 35min ahead of the second woman outright, amateur women's leader Daniela Erni Ruoss (2XU). She remains in 12th position outright with a gap of just over three hours to overall leader Bouchard.
The riders will leave Skybury Coffee and head further north on a 102km marathon to Wetherby Cattle Station for Stage 6.