That's not entirely true, Australia's schools start in February so a graduating high school student will wait 8 months before being able to start. Then they wait 3 months after graduating dentistry before receiving their NDEB results so really it takes 6 years after high school to complete everything. It only really makes sense if you know you won't get in after 4 years of undergrad and you would have to complete additional schooling for a chance at a Canadian school, but no one knows that in high school. I would recommend working hard in undergrad and trying your chances in Canada.