As others have said—many of them not even from God’s Own Country (but certainly honorary Aussies, one and all): Vegemite is ace.
I’ll add a couple of observations:
There are certainly cultural predilections in what food you end up eating by default: what a culture happens to have come up with in cuisine determines what food you’re likelier to be exposed to within that culture, and it takes a conscious effort to snap out of that predisposition. Americans like their bread sweet, and they like sweet things on bread. So they aren’t brought up to eat lightly umami/salty spreads on their bread, like Aussies are.
And Aussies are, because Brits were: Vegemite was an answer to Marmite, as many have pointed out. In fact, the original branding of Vegemite was Parwill: “Ma might, but Pa will!”
Not that Aussies are immune to ludicrously sweet toppings on bread: Fairy bread is actually a thing.
So what is it about Marmite that’s appealing culturally to British culture? Dunno, but I suspect the precedent of chutney had something to do with it.
The other thing is that Vegemite is not universal in Australia, treasonous though it be. My parents being immigrants, they wouldn’t go near the stuff, so I was not brought up with it. (I had Taramasalata on bread instead. Also savoury.) In fact it was only in my late twenties that I tried the stuff; some acquaintance had friendly advice about visualising oysters when I ate it (for the saltiness, I guess).
It is now my default breakfast out.