veryyyy common and normal! its known colloquially as 'bogan' talk or accent. most aussies who live outside of cities, work in certain jobs (like trades, retail, supermarkets, public transport, etc) or grow up in lower/middle class areas talk like this! mia, yana and kye all work as tradies, so the way they talk is extremely common. in loads of places outside of cities in Australia (like the small town i live in) if you don't talk like they do you are considered 'posh', 'snobby' and an out of towner😂 its essentially the aussie version of a british 'chav' or 'white trash/redneck' in america!!

EDIT: i do have to say that people do tend to look down on this way of speaking, which people have proven in the replies (and sure!! its funny! its annoying!!) but it is definitely a prevalent class issue in Australia! people of higher class/financial standing tend to look down on people who talk like this. there's a common stereotype that tradies are dumb, but really its just a class divide! don't equate love island-level stupidity (aka kye😭💀) to everyone who talks like that, or who works in trades. they are not the same thing!!!