Kraft apparently dropped their lower sodium Vegemite ("My First Vegemite") last year (2012) since it wasn't selling well. It's not listed on their Australian Web site. That's probably why you can't find it. You can ask the folks at aussiefoodshop.com what they know about it.
Not that it's the same, but I don't think there's a lower sodium Marmite either. Supposedly there's a lower sodium version of Cenovis (the Swiss yeast extract spread), but I can't find it online.