You might notice a images of kangaroos and/or koalas in the selvedge area of Australian stamps. 

 

It is the method used by Australia Post since the late 1980s to indicate the number of the reprint

 

So, 1 koala is the first reprint up to 4 koalas for the fourth reprint

 

The fifth reprint is 1 kamgaroo, the 6th is 1 kangaroo plus 1 koala

 

In the case of the sheet stamps, the koalas/kangaroos are in the selvedge (the edge strip). In the case of roll stamps, it is printed on the back of the paper.