Why is the kangaroo and emu on the Australian 5c coin?

The normal non-commemorative Australian 50 cent coin has the kangaroo and emu on the reverse as a part of the Australian Coat of Arms.

Here's the full list of coins, identifying the reverse design:
1c: feathertail glider (no longer minted)
2c: frill-necked lizard (no longer minted)
5c: echidna
10c: superb lyrebird
20c: platypus
50c: Coat of Arms with kangaroo & emu (non-commerative coins)
$1: five kangaroos (non-commerative coins)
$2: aboriginal elder

The obverse design has the Queen.