Those titles guarantee Australia one start in the men's and women's triathlons at the Olympics and also go a long way to ensuring a full team of three apiece.
Australia, a long-time triathlon power, only managed to secure two starts in the men's race at the Beijing Games and one of the factors was that New Zealand had won the Oceania title earlier that year.
Four years later, the Australian men have again looked more vulnerable than the women in triathlon's nation rankings ahead of the Games.
Indeed, Sexton said Saturday's win meant more to him because of the automatic national berth than it did for his own form and selection hopes.
"We came here as a team, to make sure an Australian won," Sexton said.
"It was a team effort to get one of us across the line first and I was lucky enough to make the most of the opportunity."
Sexton said the Australians controlled the Olympic-distance race as much as possible.
Knowing the locals looked stronger in the 10km run, they made sure no breaks went clear during the 40km cycle.
Sexton was near the front after the 1.5km swim and was in a group of about 15 that had the lead going into the run in the hot and windy conditions.
Australia's Courtney Atkinson and New Zealander Kris Gemmell made the early break, but Sexton quickly ran them down and then went clear for an impressive win.
Wearing hot-pink running shoes, which Sexton said he was saving for "when I knew I was going to run fast", he crossed the line in an unofficial time of one hour 48 minutes and 51 seconds.
Gemmell was 41 seconds behind in second place and young Australian Aaron Royle (1:50:07) took third, while New Zealander Jamie Dodds outsprinted Atkinson for fourth.
Sexton said he was inspired by distance runner Craig Mottram's 5000m win a few days ago at the national athletics championships.
The win was also a handy return to form after a mechanical problem stopped him from finishing last month's Australian sprint titles.
Earlier on Saturday, Jackson dominated the women's race to win comfortably in 2:02:47, 50 seconds clear of compatriot Ashleigh Gentle.
Erin Densham completed the local podium, taking third in 2:03:53.Sexton will now try to further improve his Olympic selection chances at the World Cup round in Mooloolaba, Queensland later this month and then the world championships series opener on April 14 in Sydney.
Jackson will bypass Mooloolaba and concentrate on the Sydney race.