I'm sure one of our friends down under will jump in on this.
Vegemite is indeed a yeast extract, very savoury product which has a strong taste indeed.
I grew up in England and there we had an almost identical product called Marmite.
It wouldnt surprise me if in fact they are the same with Vegemite being the Aussie name.
Great stuff !    Everytime I'm in Australia,  I always use Vegemite on my toast in the mornings!
I havent found it here in the USA but Im sure it is available.     I regularly use Bovril though which is a bit similar when spread on bread although it is a beef extract whereas Vegemite and Marmite are purely vegetal I believe.
Keeps me in touch with my childhood and the old country.
I always have some with me when I go camping in my Westy  (Mandatory Vanagon content) !!

Steve Cotsford