My uncle played in Bullamakanka, so I can confirm that the 'mull up' lyric does indeed refer to the smoking of marijuana. 'Mulling up' is act of chopping up a marijuana bud and mixing it with tobacco, prior to rolling it in a cigarette paper, or alternatively smoking the 'mull' through a water pipe or 'bong'. It's a social thing where you sit down, relax and chat while someone prepares and then offers around the smoke, hence "later on we'll settle down and mull up on the porch".

My uncle and most of the guys in the "Bullas" lived in the hills amongst the bush just inland of Tweed Heads, NSW, a place that is quite laid back, where music and pot smoking is part of the culture. I think it's good that they modified the lyrics to impart a little of what living in the bush meant to them.

Living in rural Victoria as a kid, I had no idea what 'mull up' meant and neither did anybody else. Grandma knew, but just avoided the question, and as a scout leader got her boys to sing 'later on we'll settle down and drink mulled wine on the porch'! I asked my cousin, down on a rare interstate visit, what it meant and he said 'mixing up cigarette smoke' and 'something that adults did'. There it stayed until sometime in my 20's when I found myself up in the NSW hills, at my cousin's place, sitting on the porch in the evening, watching the wombats come out to play, while he... mulled up. It was one of those moments.