Trees are a valued part of New Zealand properties, but a tree in poor health or structural decline can pose a serious risk to people, buildings, and neighbouring land. Knowing what to look for can help you act early — before a branch comes down in a storm or a root system undermines your foundation.
Why Tree Safety Matters in New Zealand
NZ's variable climate — from the wet, windy winters of the South Island to the humid summers of Northland — creates conditions where trees can deteriorate quickly. Storm damage, root disease, and fungal infections are all common. Under the Resource Management Act (RMA) and local council rules, some trees are protected by a Notable Tree schedule or District Plan, meaning you may need consent before pruning or removing them. Always check with your local council before doing any significant work.
WorkSafe NZ also notes that falling trees and branches are a genuine hazard — particularly during high winds — so assessing risk proactively is important for any property owner.
Key Warning Signs of a Dangerous Tree
You don't need to be an expert to spot many of the early warning signs. Walk around the tree and look for the following:
Structural Problems
- Significant lean: A tree that has developed a noticeable lean — especially if it has shifted recently — may have compromised roots or a weakened base.
- Cracks or splits in the trunk or major branches: Deep cracks, especially those running vertically or where two stems meet (called a co-dominant stem), are a serious red flag.
- Hanging or dead branches: Known in the trade as widow-makers, these can fall without warning.
- Exposed or damaged roots: Roots that are cracked, rotting, or have been cut by earthworks can destabilise the whole tree.
- Soil heaving at the base: If the ground around the base is lifting or cracking, the root plate may be failing.
Signs of Disease or Decay
- Fungal growth: Mushrooms, conks (bracket fungi), or shelf fungi growing on the trunk or at the base are a strong indicator of internal decay. By the time fungi are visible externally, the decay inside can be extensive.
- Cavities or hollow sections: A hollow trunk or large cavity in a branch significantly reduces structural strength.
- Discoloured, wilting, or sparse foliage: Leaves that are yellowing, browning, or failing to appear in spring suggest the tree is under stress or dying.
- Bark abnormalities: Peeling, cracked, or sunken bark (called cankers) can indicate fungal or bacterial infection.
- Oozing sap or resin: While some sap flow is normal, excessive or discoloured oozing can signal disease or pest damage.
- Deadwood throughout the canopy: A significant amount of dead branches scattered through the crown is a warning sign the tree is in serious decline.
Common Tree Diseases in New Zealand
| Disease / Issue | Symptoms | Trees Commonly Affected |
|---|---|---|
| Armillaria root rot | Mushrooms at base, yellowing leaves, dieback | Many species |
| Phytophthora root rot | Wilting, root decay, collar rot | Avocado, conifers, natives |
| Dothistroma needle blight | Brown banding on needles, needle drop | Pinus radiata, other pines |
| Canker diseases | Sunken, discoloured bark patches | Fruit trees, ornamentals |
| Myrtle rust | Yellow/orange powdery spots on leaves | Pōhutukawa, mānuka, feijoa |
Myrtle rust in particular is a biosecurity concern in NZ — if you suspect it, report it to BRANZ or the Ministry for Primary Industries (MPI).
Assessing Risk: The Whole Picture
A single warning sign doesn't always mean a tree is imminently dangerous — context matters. Arborists use a risk assessment framework that considers:
- Likelihood of failure: How structurally compromised is the tree?
- Likelihood of impact: Is there a target nearby — a house, driveway, play area, or power line?
- Consequences of failure: A large tree over a bedroom is a very different risk profile to one in an open paddock.
This is why a professional assessment is so valuable. An arborist can probe for internal decay, assess root health, and give you a clear picture of the actual risk level.
What You Can Safely Do Yourself
As a homeowner, you can:
- Visually inspect trees regularly, especially after storms
- Remove small, low branches (under about 50mm diameter) that are clearly dead, using proper pruning technique
- Keep records of any changes you notice over time
- Clear debris and check for new fungal growth after wet weather
For anything involving climbing, working near power lines, or removing significant branches or whole trees, always engage a qualified arborist. Tree work is physically dangerous and requires specialist equipment and training.
When to Call a Professional
Call a qualified arborist promptly if you notice:
- Any of the structural warning signs listed above
- Fungal growth on the trunk or roots
- A tree that has shifted or leaned after a storm
- Significant dieback in the canopy
- A tree near a building, power line, or area where people gather
Look for an arborist who holds a New Zealand Certificate in Arboriculture or is a member of the New Zealand Arboricultural Association (NZAA). For tree work near power lines, the lines company must be notified and specific safety distances observed under WorkSafe NZ guidelines.
You can find qualified local arborists in your area at tradedirectory.co.nz.