New Zealand is blessed with more beautiful landscapes than it might even be possible, or desirable, to preserve in public ownership. Fortunately, they do not all have to be a public expense. We are also blessed with an institution called the QEII National Trust which allows private owners to place a legally binding covenant on the title to the land, protecting its natural values forever.

The generosity of owners who do this goes largely unnoticed. They deprive themselves of some of the land's resale value and additional income from potential developments. A study by Waikato University's business research institute has calculated that the 4300 covenants in force, covering a total of 180,000ha throughout the country, have a total "opportunity cost" (lost development value) of between $443 million and $638m in total, or $105,000 per covenant.

On top of that the landowners maintain native species and environments at their own cost, which the study has calculated to amount to $25m a year. They do so for no return except the satisfaction of enjoying the land in its natural state and knowing their efforts and expense cannot be destroyed by a subsequent owner. The QEII Trust undertakes to monitor the land for the life of the covenant.

The study found that more than half the areas under covenants would have alternative uses such as grazing, exotic forestry and even residential development in some places. The sacrifice of those possibilities for the sake of preserving native bush and grasslands and unblemished lowland scenery is a priceless gift to this country. If it were not for the landowners' generosity many of these estates would have to be purchased at public expense and maintained by the Department of Conservation, which already has more than enough to do on a budget that is perpetually stretched.

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The trust has been operating for four decades and this study is the first time it has properly assessed the value that covenants represent. It commissioned the study to measure the cost-effectiveness of conservation activity it arranges. The results will help make best use of a newly established Stephenson Fund for Covenant Enhancement which will provide $150,000 a year for important projects. Gordon and Celia Stephenson registered the first open space covenant with the new trust in 1979.

This is the trust's 40th year. It is an anniversary the country should celebrate.

Last year the trust won a crucial court case when a covenanted block of land on the Coromandel was sold after the owner's death to a property developer for $540,000 who promptly advertised the site for $3.8m claiming the covenant could be removed. After two hearings in the High Court and the Court of Appeal, the trust obtained a ruling that a technical error in the covenant's drafting did not invalidate it and the original owner's wish had to be honoured.

This method of preserving valued property in its natural state has proved so popular that the trust receives more offers most years than it can afford to accept.

It is a fine way of building a national heritage and needs to continue forever.