What is a vegemite sandwich???

I was just listening to some classic 80's music and Men at Work and in the song Down Under they sing, he sat down and had a vegemite sandwich. What is that?? Who from down under knows what this is, maybe Willows?? I have never heard of it.

I didn't find the right solution from the Internet.

References: http://www.debate.org/forums/miscellaneous/topic/103299/

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