I like this article on house prices because the last couple of paragraphs tells you clearly why you get differences between QV and the real estate peoples numbers.
QV's index construction is interesting. We took a look at www.qv.co.nz to check ou the index and how it works - its much better than tracking 'average' house prices. You can register as a casual user, and check it out for yourself. But the following is helpful:
Why does Quotable Value recommend using the QHPI?
Quotable Value (QV) recommends using this index when commenting on sales trends, because it reduces the impact of extra-ordinary circumstances and low volume of sales, thereby providing a more robust indication of market house price trends.
There is a weakness in using average sales prices as a measure of value in the property market. Average Sales Price can be a poor indicator when the sales volumes are low and particularly when the sales prices vary significantly. With a low sales volume, a few sales of very high price properties can significantly impact the “average” price.
QV recommends the reporting of index movement rather than average sales prices.
Freehold open market sales are included.
The price to value ratio for each sale using net sale price and the rating valuation are calculated.
The price to value (p/v) ratios for the Territorial Authority are summed and divided by the number of sales to calculate the average p/v ratio.
The percentage change between the current average p/v ratio and the previous period’s average p/v ratio are calculated.
This percentage change is used to calculate the current period’s index for the Territorial Authority.