Why was billy tea called tea?

It's a bit of a misnomer! "Billy tea" isn't actually tea, but rather a boiled water beverage typically made with tea leaves but can also include other ingredients like herbs, fruit, or even just sugar. It's called "tea" because of the use of tea leaves, but the method of preparation and the resulting drink are distinct from traditional tea brewing.

The name "billy tea" is believed to come from the billycan, a metal pot used to boil the water. The billycan was a staple of Australian bushmen and miners, and so the name stuck to the beverage made in it.

So, while it's called "tea," billy tea is more about the method of preparation and the cultural significance associated with it than the specific ingredients used.