A billy is the tin pot that you boil water in when you are camping, you can make tea or coffee in it too if you wish. In Oz we use the term "to boil the billy" when we really mean "to heat water in the electric jug"! Especially if you grew up in a country town like I did, and not in Sydney where I now live (and I want to leave too and go back to the country.....)
Cheers
JennieG - off to boil the billy - it's coffee time!