So it's time to extend that requirement across the nation, and perhaps even make it 40 per cent. But the Reserve Bank could go even further.
To cool the overheated housing market in the the UK, the Bank of England told lenders to restrict loans to 4.5 times a person's annual income. It's a simple solution that restricts what people can pay for a home.
Not only that, borrowers will likely be in a better position to survive financial shocks, such as redundancy, unexpected bills and rising interest rates. They'll also have more money to spend in the wider economy.
But can it work here? The grandly named Property Institute claims that if New Zealand was to follow the UK's lead, then it would "kill the market dead". That is, of course, rubbish. The market is not dead in the UK or anywhere else where sensible lending is practised.
One of the problems is that because house prices are rising so fast, lenders are on a good bet when lending for property purchases. Even if our Reserve Bank Governor didn't care about house prices, he must have concerns about household debt levels.
Average, hard-working Kiwis can't compete with cashed-up foreigners (who may opt to leave a property empty) or investors leveraging one property against the other to build their portfolio.
Something will have to give if Auckland wants to retain the young people it needs to work in the city.
For an insight into some of the social implications of unaffordable housing, look at the excellent nzherald.co.nz/hometruths.
Nationwide rises
Economists at Westpac are predicting that property values across New Zealand will rise 11.5 per cent this year.
They were surprised by "the power of the housing market's rebound" and that rising house prices are becoming widespread.
Rising household borrowing, they say, strongly suggests that low mortgage rates are a powerful driver of the market.
One almost wonders if raising mortgage rates is an option.
