Australia has some really cool words. It is not always clear where they come from. Words like Bogan, Sheila, bubbler are just so unique to Australia. I was wondering why does Australia have this slang? when did it become so widespread (Australia is HUGE) and how did it spread?
This isn't really a question for historians as it is much too general. Australian slang has many different sources. Some of it is deliberately obscure, like rhyming slang, some of it is quite old and although well understood is considered slightly out of date and some is regional. There is no common rule for slang formation and various reasons for it's distribution. While Australia is large, Australians are quite mobile and quick to pick up and use slang. If you are interested in particular terms:
BOGAN means a male person, usually from the low income outer suburbs, with a particular style of dress such as flannelette shirt, mullet haircut, moccasins or work boots etc. Usually implies the person is uncultured or unattractive. Is used in a similar way to the term "Westie" which means someone from the western suburbs, particularly in Sydney or from somewhere further west. The term Bogan may be a play on Westie as Bogan Gate is a small town a long way west of Sydney. Bogan is a fairly recent slang term first appearing in the 1980s when it was regularly used on the television Comedy Show spreading the term widely.
SHIELA means a woman. It comes from an originally Irish girls' name (sometimes spelt Shelagh). It is known in Australia as a general term for any woman of Irish origin in the 1830s and expanded to mean any attractive woman. It is used Australia wide. It is now considered old fashioned and women may find it mildly derogatory.
BUBBLER means a drinking tap or fountain. In particular the type in schools where the tap is spring loaded to cut off as soon as it is released and the water is released vertically (upwards) so the drinker bends over the tap and puts his/her lips close to, or touching the nozzle and drinks directly from it. The word is descriptive. Where the water pressure is low, the water just bubbles from the nozzle. This is regional slang appearing throughout N.S.W. but rare in other states.
If you are interested in how Australian slang spreads, the Macquarie Dictionary has the Australian Word Map, a useful resource listing regional slang, with where and how it is used https://www.macquariedictionary.com.au/resources/word/map/.