They are created when landowners voluntarily enter into a binding legal agreement with the trust that preserves a piece of land in perpetuity, safeguarding it from development by future owners.
Mr Guild said the trust budgeted for between 3,500 and 4000ha of new covenant land each year, and looked at around 30 fresh proposals every two months.
But establishing each one - a process that involved reviewing the offer, then surveying and fencing off the land - cost an average $25,000, while the trust also assumed the cost of ongoing monitoring.
"The demand is there, but we can't fulfil that presently because we don't have the resources," he said.
"You could say that it's a nice problem to have, but it's also disappointing for people who say I'd like to protect this, and we are saying you might have to wait for a while."
The trust received about one per cent of the Government's conservation funding and last year received a $1 million funding boost.
Mr Guild said the trust regularly accepted bequests and donations, and was also working to address its funding issues through sponsorship and commercial partnerships.
Dr Marie Brown, the senior policy analyst for the Environmental Defence Society, saw the issue as pressing because protecting private land is important for curtailing biodiversity loss.
Covenants contributed most when they protected ecosystems not well represented on Crown land and were managed effectively to address threats such as pests and weeds.
"In fact, our most vulnerable terrestrial biodiversity is mostly on private land, so it needs that focus, but what we've got instead is a situation where the trust is not getting the level of funding they need just to meet the demand from keen landowners."
She noted one of the key points in her newly-released book, 'Vanishing Nature: facing New Zealand's biodiversity crisis', was that there is a "massive gulf" between the money given to conservation and what was required.
"The costs of protecting and maintaining habitats on private land can be significant, and New Zealand has a paucity of incentives to reward landowners for this.
"The important work of covenanting bodies like QEII in building positive relationships with landowners and supporting them in their voluntary conservation efforts is essential for New Zealand if we are to meet international biodiversity commitments."
Conservation Minister Maggie Barry told the Herald she was happy to meet with the trust to discuss any issues it was facing.
Registering covenants could be a complex and lengthy process because of the consultation required and the need for agreement from all interested parties, she said.
"There is no statutory time frame for completing registration, and I'd like to commend the trust on how many they are able to get done each year."
Ms Barry acknowledged covenants were an important way of protecting private land and bringing it under active management.
"They complement other protection methods such as acquisition by the Crown through the Nature Heritage Fund or Kawenanta agreements for Maori-owned land."
4000th covenant an "honour" for Waikato dairy farmers
Keith and Margaret Ormsby have spent decades pouring their hearts into their little corner of the country.
But long after the Ormsby family leaves its dairy farm, near Otorohanga at the foot of Mt Pirongia, they'll know their efforts will live on.
At a Waikato function, the couple yesterday celebrated registering the 4000th covenant with the QEII National Trust, which oversees around 180,000ha of covenants across New Zealand.
Co-incidentally, the 4000th covenant lies not far from where the first ever space was registered 36 years ago by a key founder of the trust, Gordon Stephenson, and his wife Gordon Stephenson, and his wife Celia.
The Ormsby's land is now home to four different covenants, protecting around 15ha of forest remnants and critically under-protected wetland areas.
Developing the farm had been a labour of love for Mr Ormsby, who recalled living in the corner of a manure shed with his wife while the couple readied the land for dairy farming.
Since then, they and their children have planted around 27,000 native plants and plan to plant thousands more to restore and enhance their covenants and other natural areas on the farm.
Mr Ormsby felt "very lucky" to have registered the milestone covenant, which he said symbolised the efforts of the thousands of other landowner like him.
"It's a significant decision for each and every one of them, and we are proud to be virtually representing them."
