Is this anything really new? Hasn't this gone on for a long time depending on tbe economic situation between the two countries?
In fact, even in good economic times New Zealand still had/has also lost populations to Australia. Probably akin to Canadians going to the US for work and immigration. One additional factor is that once you are a New Zealand citizen the minute you step out of the plane in Sydney you can go rush in to apply for any jobs courtesy to the Common Labour Market polocy. (Although you can't share Australia's social welfare goodies - you need to buy health insurance etc).
The only thing that keeps expat New Zealanders in Australia at 455,000 rather than 2.5 million is it takes 3 hours to fly to Australia's East Coast from NZ.