Keith - I can no more explain the aussie colloquial terms any more than you can explain yours...... Dude??
The adding the "O" is typical - Dave does become Davo, John - Johno Jack - Jacko but Keith wouldn't. It could become "Keitheee" with a guttural eee but not all the time, it depends on whether it's your shout or not
It's your shout means it's your turn to buy the next round of beers. Beer makes up 50% of all alcohol consumed, wine 30%, spirits 20%.
A great deal of our terms originated in the gold fields where the Irish and British had an influence - cockney from the brits plays a big part.
Fair Dinkum actually comes from the Chinese who made up 20% of the gold field era. (yes - we had a gold rush too, remember the last scene in Bush Cassidy where they decide to go to Australia?? ) Dinkum meant pure gold so Fair Dinkum became to mean genuine, he's fair dinkum meaning he's honest and good. Australians like to believe that they are fair dinkum.
The term "Digger" originated in the gold fields and meant exactly that - someone who dug for gold but it soon stood for our soldiers, the Diggers, from WW1 and later wars. Our vets are known as Diggers but anyone can use the term as in "How are you going digger?" so it becomes another term when you can't remember someones name, like maaate

.
Mate has many uses. The sad downward inflection maaate means "I feel for you, you didn't deserve that" whereas the straight clipped version Mate! basically means hello. Maate?? with an upward inflection signals "what the fuck are you doing??" and can be used to pull someone into line. The long low pitched maaaaate means "isn't this great, or I'd like to get into her pants"
A significant number of Australian colloquialisms are affectionate insults or backhanded compliments. A clumsy friend or colleague may be called a 'dag', 'galah', 'drongo' or 'boofhead'. We tend to shock people overseas with our backhanded compliments which are intended as affection. "Not a bad job mate for someone as ugly as you " would be a compliment on the job and your looks, well to an aussie anyway.
BTW it's QANTAS, no U - it stands for Queensland And Northern Territory Aerial Services (PTY LTD) which was purchased by the government and became our national airline. It's the only airline in the world that has never had an accident causing death. It's recognised as the safest airline. It's since been sold by the government and floated on the exchange. It's now facing a private equity takeover bid.
I hope this helps.
cheers
john