Walker looked untroubled all day, breezing through the moto rounds to start the final as the fastest seed.
"I felt really calm out of the gate and all I was thinking about was just one jump at a time and giving 100 per cent.
"My preparation this year has been so much better than last year. My mind is all about having fun and enjoying my riding and when I do that, my trainings are better and my racing is better which means everything is enjoyable," Walker said.
Petch's second-place finish was a continuation of consistent form at the Oceania Championships for the New Zealand elite champion, who won three silver medals, as an under-19 in 2015 and 2016 and last year as an elite.
Hamilton's Cailen Calken won the bronze medal in his first year in the junior elite category with Kapiti's Luey Cotton seventh in the final. Earlier Cambridge rider Rico De Anvers crashed in practice but was able to race, making it to the semifinals.
Cambridge rider Toni James and Pukekohe's Holly Woodcock-Takurua finished second and third respectively in the final of the women's 17-24 years category in a closely-fought final, while North Harbour's Thomas Webb was fifth in the final of the boys' 11-12 years cruiser class.
The men's elite final was won by Corey Frieswyk of Australia. He claimed his maiden Oceania crown by edging Australian-based Kiwi Brandon Te Hiko by five-thousandths of a second in the final.
It was not such good news for New Zealand rider Maynard Peel, a former junior world champion, who suffered a muscle strain in practice. While he decided to race, he was unable to make it out of the moto stage.