This term is used a bit like bastard, to mean a disreputable person.  But if more Australians understood this term they might use it less often.

Used.  Pre-loved, pre-owned, handed down.  Like the English language which was bequeathed by the British to their ungrateful colony.

The upright crest (shag) on a cormorant (shag) brings readiness for this activity to mind.

Sheila was a common name for a woman in the early days of European settlement, being based on Sheilagh, an Irish name. Some people still call women sheilas or birds or chicks but these days if you want to refer to women casually you would probably just say girls.

It'll be all right.  Comes from She'll be nice, rhyming with Apples and spice, so She'll be apples.

Hair down there.

Treat.  To pay for someone else. "Whose shout is it?  Whose turn is it to pay?" "It's my shout."  From the old days when you shouted for service at a pub.

Smelled spelt (spelled) the British way.

Sausage.

Sorted out, organized, taken care of.

Tease; attempt to agitate someone.  "I was just stirring you."

A storey is a level in a building.  So a three-storey building will have a top floor which is a third floor in America but a second floor in Australia.

Exclamation of surprise, originally God's truth!

Exclamation of emphasis.  Comes from May God strike me dead if I am lying.

Australian.  When read with an Australian accent, Strine sounds like Australian to us.  So... uh... I might have appropriated it for my pseudonym.

Erred.

Baffled, as if you have been plowing and have come up against an intractable object like a tree stump.

Superannuation, Australian retirement savings, equivalent to 401k plans in the US and private pensions in the UK.

Originally, sweet as pie.  Now means awesome.