An urgent warning has been sounded for the trees remaining in Auckland's concrete jungle after researchers found there's scant protection left to save them from development.

A study shows the Auckland isthmus has just 6 per cent of its urban forest left, and of that, well over half stands on private land. Only 15 per cent is protected through Auckland Council's Schedule of Notable Trees - the only remaining tool for tree protection after changes to the Resource Management Act in 2012.

The researchers believe their stocktake is the first of its kind conducted for central Auckland, and they expect tree numbers will have dwindled further when they later carry out a follow-up review.

Study co-author Dr Margaret Stanley, of the University of Auckland, told the Herald the city's urban forest was in "a really urgent state of play".

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As the city's housing crisis worsened, spurring on more intensification, more trees would be lost unless safeguards were introduced.

Trees provided a wide range of benefits, providing habitats for a wide range of wildlife, combating pollution, reducing stormwater runoff, and, as recent studies have shown, boosting the physical and mental wellness of residents who live among them.

Yet Auckland was lagging behind the rest of the world in preserving them, with most cities having tree cover targets of up to 40 per cent because of their recognised value.

The study found the spread of protected trees across the city was highly uneven, with older suburbs having relatively high numbers of protected trees, while other areas have few.

The type of species protected through the schedule was also skewed towards popular species - some on the protected list were even weeds or listed elsewhere by the council as pest plants - while a threatened species native to Auckland, tawapou, was represented by just one tree.

"The study shows the schedule is failing to adequately protect unique native tree species and we need to do much better if we are to protect what is left of the city's urban forest," Dr Stanley said.

Dr Marie Brown, lead author of the new book Vanishing Nature, said people often thought urban biodiversity was less important than that in the wild.

"But in fact a majority of our acutely-threatened land environments are ... within 20km of an urban centre in New Zealand - this means it is vital to safeguard what remains."

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Charmaine Wiapo, who oversees a Ngati Whatua-led project to return an area of land at Bastion Point back to native bush, said the urban forest had become "very fragmented".

Because of this, the 200,000 trees so far planted over about 30ha had been aligned to link up with tree corridors elsewhere in the city, providing food stock for birds such as tui and kereru that flew between them.

Forest and Bird was similarly working on a "wildlink network" that would connect habitats in trees in the urban area with those in the Waitakere and Hunua Ranges and the Hauraki Gulf Islands. The group was also pushing to get trees that had ecological value included in the Proposed Auckland Unitary Plan.

Deputy mayor Penny Hulse admitted general tree protection rules had taken "a bit of a hammering" over recent years, and agreed more could be done.