Borrowers disposable income starts to fall after two years of high weekly levels; wide variance between cities

For most people, the biggest financial transaction they will ever do is to buy a house.

The first one is pretty scary, and requires a leap of faith that you will be able to handle the obligations involved.

The reason it is so scary is that you are signing up to borrow a huge sum over a very long time. And that loan is personal. It might be secured against the house, but it is your obligation no matter what happens to the house or to you. People in Christchurch know how that works. There is no 'jingle mail' in New Zealand - you can't just walk away from a New Zealand mortgage by handing the property back to the bank. The mortgage debt will follow you.

Only bankruptcy will 'save' you, and that is not a place you want to be.

So when you have that mortgage, it makes sense that making the repayments becomes about the highest priority for the money you earn.

Conceptually at least, it is a higher priority than food; you can cut back on the cost of food and the type of meals you eat, but there is no cutting back on the mortgage payment. It is what you agreed with your bank.

Given that your mortgage payment is your number one spending priority when you buy a house, another way to look at its affordability is to recast your income after making that payment.

Your take-home pay less your mortgage payment is your disposable income. Your disposable income is what you have left to live on.

Using the Roost home loan affordability data we publish each month, we can assess where you will have the most disposable income in New Zealand, and where you will have the least.

If you find the worry oppressive, maybe you should seek out a community where you can enjoy more of your earnings on things other than your house.

Our data tracks the median incomes for both the 25-29 and 30-34 age groups, by city, and can relate that to the mortgage payment needed to buy either a entry-level first-quartile house or a median house.

If you are a one-earner household with a mortgage, you would have the highest disposable income if you buy a median house as a 30-34 year old in Invercargill. After the mortgage payment, you will have $517.85 per week for all your other expenses. 

If you are a 25-29 year old buying a first home (that is, a first-quartile priced home) in Wanganui, you will have $514.79 per week left over after paying the mortgage.

The top five places (plus one) where you keep more of your take-home pay after buying a median-priced house are these:

The top five places (plus one) where you keep more of your take-home pay after buying a first quartile-priced house are these:

At the other end of the scale, Queenstown is the toughest town. As at December 2012 you will spend all of your income except $77.28 trying to pay the mortgage on a median priced house while earning a median income in the Central Otago Lakes district. Basically buying a house is impossible there on one income if all you can put down is a 20% deposit. Its even worse getting started out as a 25-29 year old.

While better, it is tough in Auckland as well. You will have less than $300 per week left over on a 30-34 year old median income, and less than $200 in disposable weekly income if you are in that 25-29 year old group. In Auckland you are basically shut out unless you win lotto. Even a two-income household sets up a choice between "house-or-children"; the horse has bolted for young people on median incomes in Auckland.

Here are the top five places (plus one) which are the worst places to buy:

On average, across the country, as at December 2012 you can expect to have $353.64 in after-tax, after mortgage payments when you buy a median priced home if you are in the 30-34 age group.

If you are in the younger 25-29 age group buying a first quartile home, you will have a disposable income of $382.00. Your income pay be lower, but nationally first quartile homes cost much less which is why they are a good first-home-buyer option.

Here is the full list.