Via Craig Newmark, here is one short article. the conclusion:
"Prosperity does not come by accident,” Mr Rennie said. "Australia has a stronger political consensus around policies for growth, which contributes to investor confidence.”
Sorry but I can’t buy it. Throughout the 1990s New Zealand economic policy probably "led" Australian policy, yet Australia has gained on New Zealand since that time. I’ll instead cite booming resource prices (there is more gain in selling minerals to China than agricultural products), economies of scale from having a larger country, and most of all the Kiwi brain drain. In percentage terms, many more of the smartest New Zealanders leave their home country — often for Australia I might add — than vice versa.