As an Australian who has been eating avocado on toast (not "an avocado toast", would you say "a peanut butter toast"?) since infancy, watching this meme develop has been a bit odd. So, in recent years there has been a trend of "smashed avo" in Australian cafes, which was generally avocado, toast and something else like eggs or haloumi or feta etc... usually with salad or dressing. Some out-of-touch moron went on TV saying that if young people stopped spending $20 on smashed avo and $10 on coffees every weekend they'd have no trouble affording homes in some of the most overpriced housing markets in the world. In the transition to global internet meme these bits of context have been lost and it's turned into something about how avocado on toast is a new dish that millennials invented and because avocados are expensive they can't afford houses, which is a slightly stupider argument than the original one about getting brunch at cafes.
Anyway, vegemite and avocado on toast is an amazing combination.