Oz Girl has given you the dinkum answer. She's true blue.
But fair drinkum has a few nuances.
"Fair dinkum!" said in a disparaging tone can mean, Is this bloke serious? Do you expect me to believe that? Or, as my Scots friend might put it, "D'ye think I've a zipper up the back of me heed, Jimmy?"
"Fair dinkum!" said in a tone of wonder is likely to mean, "You little ripper! That's great mate! You really did it."
But a smelly fair dinkum? You're confused. You're thinking of a dunny which translates to British English as a khazi and American as a john. Here's an example to illustrate the usage:
A silly old fellow from Ryde
Fell into a dunny and died.
Later his brother
Fell into another,
And they lay there interred side by side.