I've got 5 Aussie coins with different stampings as follows:
1997 $1 (Smithy) with 0 stamped on both sides
1975 50¢ with an E stamped on the obverse
1967 20¢ yesterday with a 0 stamped on the obverse
1968 20¢ with U stamped on the obverse
1998 $2 with upside down 3 stamped into HM’s neck
With the first 4, all are stamped to the right of HM's neck. I'm not sure if the 0's are letters or zeros.
On the Australian Coins, Tokens and Banknotes Forum there's been a bit of speculation lately about who's responsible, but nothing concrete has been established.