Numismatic value for past year australia kangaroo gold coins?
 

The australian kangaroo are my favourite bullion coins due to their design which changes every year. I am planning to collect them instead of the boring eagles/maples since they all have about the same premium. Do kangaroo coins have any numismatic value? Are old dates kangaroo worth more (or less) than current year kangaroos? In general, with other mordern bullions, it seems that the value of the gold coin actually goes down as it ages.
Thanks for any advice.