At this stage, the 'best' way I can see in order to get Nuketown 2025 and be as 'legal' as possible while paying the least amount is to get a US member (or a member whose CC is accepted in the US store) to gift it to you.
From: 202970 Search Results /au SteamPrices.com
AU Store: AUD$99.99
US Store: USD$59.99 » ~AUD$59.99 (-40.00%)
UK Store: £39.99 » ~AUD$63.14 (-36.85%)
I am pretty sure after the Russian MW3 key debarcle, Acti will be keen NOT to let that cat out of the bag again. I would not knowingly be purchasing a Russian key at this stage.