I don't what there decisions are internally, but things that strike me about Australia is :
1.The Australian dollar is going through the roof with the commodities boom supporting China. No point in having your cost centre exploding. 2. Australia's GST system can potentially provide advertisers with a 10% disadvantage over other regions - so no point in locating there to get inflated invoices and tax bureacracy management costs. 3. Singapore's corporate tax rate is half of Australia's 4. 27 million English speaking Aussies and Kiwis in the region approx, versus an Anglo/Chinese speaking population, perhaps geared for Chinese servicing of 1.6bn person economy.
Australians are moving offshore in droves with retailing, servicing and manufacturing benefits of 40-50%.
These are just a few of my reasons for Google moving - but i know nothing of the inner workings. Just a hunch.
I have a feeling that Google is looking at financial steps the Irish government may soon take to escape the PIGS acronym (Portugal, Ireland, Greece, Spain) countries failing under the Euro---by raising taxes across the board. However, moving their offices from Ireland to Singapore is significant for Google and will impact a few other countries besides Australia.