He's not Don Bradman, you know.
Australian colloquialism, used when explaining to someone that they are asking too much of a person. Derived from the fact that Australian cricketing hero Don Bradman could do anything*. The use of the "you know" (accompanied with a slight raising of voice tone) shows a common trait in Australian slang; turn as many statements around into questions. The common reply to this saying would be: "Yeah, I know mate"
"Hey Robbo, if ya gonna go to the pie shop, get me a steak an onion pie... oh an get us a snagger roll and a coke thanks mate. Yeah and couldja be back before the ads finish?"
"Back before the ads finish? Hey Simmo, I'm not Don Bradman, you know."
* Well ok, Don Bradman could do anything provided he had a cricket bat in his hand.