Kiwi landowners who have locked property into QEII covenants are spending an estimated $25 million of their own cash each year to protect native species and environments.

That estimate comes from a study published today by the University of Waikato Institute for Business Research, which further found the landowners, predominantly farmers, had made an overall financial commitment of around $1.1 billion to $1.3b since the QEII National Trust was established four decades ago.

QEII open-space covenants, crucial for protecting New Zealand's most vulnerable terrestrial biodiversity, which is mostly found on private land, are legally binding agreements that are registered on the land title and protect the associated land and its natural values forever.

The covenanting landowner and subsequent owners retain ownership of, and management responsibilities for, the protected land.

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The QEII National Trust is the perpetual trustee to ensure the covenant aims are achieved by monitoring the covenant and advising and supporting the landowner.

The 4300 covenants now protect about 180,000ha, the study found, and an average of two covenants have been established every week since 1977.

The area being protected by covenants was still growing, and a further 115 are on track to be registered this year.

The analysis further showed how more than half of covenanted areas would have had an alternative economic use, such as grazing, residential development or exotic forestry, that would not happen once the landowner covenanted the land to protect its natural values.

The total estimated maintenance expenditure on covenants was $25 million a year and, calculated over 30 years, had a net present value of $387 million; equating to an average of just under $6000 for each covenant every year, or $150 per hectare of covenanted land.

Conversely, total "opportunity costs", or the loss of potential income from other alternative uses, associated with the covenants was estimated to be in the range of $443m to $638m.

That equated to about $105,000 per covenant or $2657 a hectare of covenanted land calculated over 30 years.

The study was commissioned to provide a framework for estimating the cost-effectiveness of conservation activity arranged by the trust.

''This is the first time the National Trust has undertaken research of this nature and it has given us a clear indication of the huge investment landowners around New Zealand have made in covenants since the National Trust was set up 40 years ago," trust chairman James Guild said.

"With the release of this report we acknowledge the hard work, philanthropy, generosity, and passion of the thousands of landowners who have voluntarily elected to covenant special places on their land with the National Trust."

The study's release co-incided with the launch of the Stephenson Fund for Covenant Enhancement, setting aside $150,000 a year to support covenantors with strategically important projects.

It can also be used to help covenantors with recovery plans for their covenants after being hit by extreme natural events, or if they were facing other challenges such as large financial burdens or health issues.

The fund did not draw on taxpayer money, but came from other sources such as bequests, donations and revenue from the National Trust's investment portfolio.

According the trust's last annual report, current baseline resourcing meant it could afford to establish about 110 covenants a year, which is fewer than the demand from landowners.

When the trust's 4000th covenant was opened in 2015, an Environmental Defence Society researcher voiced concern over whether its annual Government funding of $4.27m was enough to help it protect important biodiversity on private land.