There's a few companies who sell English food and provide a delivery service. I've used treatsfromhome, they're based in Melbourne but would ship to you on the sunshine coast. There's also atasteofhome as well.
Coles used to sell MyMate which was Marmite but made in NZ (I think....my brain is becoming addled in it's old age). Do a search on here as it's come up before.
I'd be happy to send you a jar from the US if you can't find it locally.
I can get Marmite and Promite at a shop across the street, but we have to bring Vegemite home with us when we visit Oz, because the US in all it's ridiculousness has decided that Vegemite is somehow dangerous or something.