Billy tea is the drink prepared by the ill-fated swagman in the popular Australian folksong Waltzing Matilda.
From the en.wikipedia.org
The song Waltzing Matilda, which is about a swagman and squatters, is probably Australia's best internationally known and most well-loved song.
From the en.wikipedia.org
Then we introduced the daring new tactic of an old man with rose-tinted glasses casually strumming a guitar, singing the great bush ballad about a swagman stealing a sheep to eat.
From the smh.com.au
More examples
An itinerant Australian laborer who carries his personal belongings in a bundle as he travels around in search of work
The Swagman Restaurant was a restaurant in Ferntree Gully, Melbourne, Australia, which opened in 1972 and burnt down in 1991. The restaurant was famous in Melbourne for its long-running television commercials, cabaret shows, and smorgasbord.
Itinerant farm worker, tramp
An unemployed man of few skills who travelled about the country on foot, carrying his few belonging tied up in a swag. These men earned a very meagre living from odd jobs, occasional employment, and country charity.
An itinerant traveler who carries all his belongings in a swag. From Bill Bryson's In a Sunburned Country: "The term comes from the rolled blanket, or swag, he carried." This may be an itinerant worker or simply a hobo.
A gunman who speaks in an Australian accent and wears a costume based on the clothes of Australian bandit Ned Kelly. An enemy of Dark Ranger.^[3]
One who travels doing odd jobs, etc.
A tramp or homeless person in the bush, who carries all his possessions in a 'swag' or blanket-wrapped bundle. Not really in use nowadays.