It's natural. Australia is a much bigger country, which has traditionally had a strong economy. This means a more buoyant jobs market and higher wages, especially for blue-collar workers and middle-income earners. The climate isn't bad either.
Things changed a couple of years ago, when the mining boom ended and the Australian economy took a turn for the worse.
Meanwhile, New Zealand has been doing just fine, so people are staying put rather than moving, and a fair few have come home.
If Australia gets back on its feet, we could see the traditional drift across the Ditch recommence.
It's true that migration numbers from Asia have also increased in the last three years, while Europe has been steady for a while.
However, the turnaround from Australia has undoubtedly been the biggest swing factor in pushing migration to the current record highs.
We all know the effects of this, most notably on housing. Demand far exceeds supply, and prices increase substantially in response. There's a fair amount of speculative heat in house prices too, but also some strong fundamental drivers.
Maybe this dynamic won't always be in place. Net migration last month was solid, but it was the weakest in nine months. Migration will likely remain solid, but this could also be an early sign it has passed its peak.
Turning to Australia, the unemployment rate fell to 5.7 per cent last month, the lowest since September 2013, which happens to be about the same time migration trends to Australia turned around so drastically. Other indicators also point to a more stable economy. Last month's NAB business conditions index posted the highest reading in a year-and-a-half.
Australia still has a few challenges to deal with, mostly mining and China-related, but also politically. However, if they do get their act together, the migration trend we are seeing at present could change markedly.
Maybe the biggest threat (or solution, depending on your perspective) to the Auckland housing problem is simply waiting for Australia to come right, and for normal service to resume.
Mark Lister is head of private wealth research at Craigs Investment Partners. His disclosure statement is available free of charge under his profile on craigsip.com. This column is general in nature and should not be regarded as specific investment advice.