In the 80s there was uproar over flogging the family silver to pay off Government debt. Now there are murmurings of discontent over flogging the family garden to pay off private debt.

Politicians have given themselves greater discretion over land purchase decisions, so the furore over selling large tracts of productive land to foreigners is unlikely to go away.

There has been plenty of debate over whether selling farm land to foreigners is a good idea or not. What hasn't been explored is why the trend is unfolding.

This situation is not unique to New Zealand. It is part of a global trend. Asian and Middle Eastern countries have been on a land buying spree in Africa for the past five years.

The size of these transactions dwarfs those in New Zealand. Five sub-Saharan countries have together sold 2.5 million hectares of land over the past five years, compared with New Zealand's 160,000 hectares.

To put it in context, New Zealand has just under 9 million hectares of productive farmland (although we don't know how much is already in foreign hands).

There are three main global drivers behind these trends. First, trade surpluses. For the past decade, Asian and Middle Eastern countries have been running big surpluses with the West. As we have been busily consuming their oil and easily broken plastic collectibles, they have been holding back on consumption themselves and squirrelling away their pennies.

Normally, when a country runs a trade surplus (sells to the world more than the world does to them), we have to buy their currency to pay them, forcing the price of their currency up.

Over time this makes their stuff more expensive, so we will buy less. But this hasn't happened.

In order to keep the value of their currencies low (competitive), they have made sure we get the money back.

One way is to lend us the money to feed our spending addictions (e.g. buying houses). Alternatively, they just send us the money straight back by buying our assets. So one reason for the land grab is Asian and Middle Eastern countries have simply taken the money we spend at The Warehouse and the petrol stations and are using it to buy New Zealand land.

The second driver is growing demand for food, particularly protein. The world's population is expected to top nine billion by 2050. All these people need to get their 2000-odd calories per day from somewhere.

This population is also growing richer - China's middle class is now more than 80 million-strong and is expected to top 700 million by 2020.

The new middle class are likely to want to eat more like our middle class - particularly more protein. Animal protein is not that easy to grow; it takes 20-40 times the amount of energy to make a kilo of beef than a human actually gets from eating the beef. This is simply the way the food chain works. It takes around 16kg of wheat to make 1kg of beef. There are serious doubts over whether the world has the resources for everyone to eat as we in New Zealand do. So protein scarcity is likely to keep pushing up farm prices.

The final driver is water and climate change. Water is a key ingredient to all agriculture, and again particularly for animal protein. To produce one litre of milk is estimated to require up to 1000 litres of water.

Water is likely to become scarcer as climate change kicks in. We have already seen serious droughts in Australia, and models predict reduced rainfall in many places, particularly in Southern Europe and the Middle East, causing fears over food security.

Again, New Zealand is blessed with this key ingredient, and in the past we have given it away freely to farmers through resource consents. Foreign buyers are now getting the benefit of that deal.

An unexpected benefit of foreign ownership might be that by having foreigners own our land, perhaps we will be able to regulate the methane and run-off pollution caused by farms without a backlash from the Neanderthal farming lobby. Foreigners don't get a vote for a start, and China is leading the world in terms of progress on emissions now, while New Zealand is distinguished by its lethargy. Maybe we could even get them to pay for their water.

In summary, the availability of cheap credit, growing demand for animal protein and fears over food security are all adding up to this foreign demand for New Zealand farms.

The only lingering question is why New Zealanders don't see the same long-term opportunity.

* Gareth Morgan is director of Gareth Morgan Investments.