Is this Australian coin rare? Has a 0 imprinted onto the back.?

It is a standard Australian dollar coin from 1984 and on the back, near Queen Elizabeth's shoulder, just below the 1984 is a 0. The 0 is stamped onto the coin, and has not been carved. The 0 itself is about 4 millimeters long and 2 and a half millimeters wide. It is also deep enough into the coin, to become notable depth. I want to know if this is rare, and if it is, how much it could possibly be worth.