AS 4970:2025: what changed, and what your council now asks for
AS 4970:2025 was published in December 2025 as a second edition, with Amendment No 1 already incorporated. It superseded the 2009 standard, and a good deal of the vocabulary changed with it. When I last checked, several of the better known Melbourne consultancies still made no mention of the new edition on their websites, and a couple still advertised the 2009 one.
What is covered
1. The change that matters: NRZ is not TPZ
Under AS 4970-2009 you calculated a radius from the trunk diameter and that radius was the Tree Protection Zone. One calculation gave you the figure you fenced to.
The 2025 revision splits that in two:
On an open site they usually come out the same. On a constrained site they often do not, because the roots are not where the formula assumes they are, or because something on the ground makes a different boundary more sensible.
The calculated figure is now where the assessment starts rather than where it finishes.
That changes how a report should read. Giving one figure is no longer enough. State the NRZ, then state the TPZ you have applied to the site, and where they differ explain what you found that made you move it. If a council later questions the zone, that explanation is what answers it.
2. DBH became DSH
Diameter at Breast Height is now Diameter at Standard Height. The measurement point has not moved. It is still 1.4 m above ground level.
This is a terminology change rather than a technical one, and it is a sensible one: "breast height" was never a defined anatomical position and it varied with whoever was holding the tape. But it does mean a report written in 2026 using DBH reads as though it was written from an old template, and council arborists notice that sort of thing.
Watch out for councils measuring somewhere else entirely
The standard says 1.4 m. Melbourne councils do not agree with each other or with the standard. Merri-bek measures at 1.2 m. Bayside and Whitehorse measure at 1.0 m. Darebin measures at 1.5 m. Yarra measures at 1.5 m, and its new local law from 1 September 2026 needs checking against the gazetted text.
On a tree with any butt flare those give materially different numbers, and a tree can be over a threshold on one measure and under it on another. Where more than one control applies, measure at both heights and report both figures. Each council page sets out which applies where.
3. How the zones are calculated
Notional Root Zone
The multiplier is unchanged from 2009. NRZ radius is DSH multiplied by twelve:
NRZ radius (m) = DSH (m) × 12
Or, working in millimetres as most people do on site: NRZ radius (m) = DSH (mm) × 0.012. Clamped to a minimum of 2 m and a maximum of 15 m.
A 500 mm DSH tree gives a 6 m radius. A 1,400 mm tree would give 16.8 m, so it is capped at 15 m.
Structural Root Zone
The SRZ is the area holding the tree up, and it is calculated differently:
SRZ radius (m) = ((D × 50) 0.42) × 0.64
Where D is trunk diameter in metres measured just above the root buttress. Minimum 1.5 m.
The SRZ is a lot smaller than the NRZ and it carries more weight. Cutting into the NRZ costs the tree some of its rooting area. Cutting into the SRZ starts to affect whether the tree stays standing, which is why the standard treats any intrusion into it as major.
Multi stemmed trees
Combined DSH is the square root of the sum of the squares of the individual stem diameters:
DSHcombined = √(D1² + D2² + D3² …)
Two mistakes are common here. Adding the diameters together overstates the figure badly. Taking the largest stem understates it. Three 300 mm stems give a combined DSH of about 520 mm. Added together they would give 900 mm.
There is a free calculator built to these formulas if you want to check a figure, including the multi stem case.
4. Encroachment is now measured by NRZ area
This is the part that changes how impact assessments read. Encroachment is assessed as a proportion of the area of the NRZ, and the bands are:
| Classification | Encroachment | What it means in practice |
|---|---|---|
| Minor | 10% or less of NRZ area | Generally acceptable where the tree is healthy and the remaining zone is undisturbed |
| Moderate | Above 10%, up to 20% | The project arborist must review the proposal and demonstrate that viability can be maintained |
| Major | Above 20%, or any SRZ intrusion | Requires a substantially stronger justification, and often a design change |
Two things to watch. The first is that the percentage is taken from the area of the zone. People routinely work it out from the radius instead, which understates the encroachment badly, because area goes up with the square of the radius. The second is that any intrusion into the SRZ at all is classed as major, however small the NRZ encroachment happens to be.
The moderate band now carries an obligation
Under the 2025 standard, moderate encroachment is not just a category you write down. The project arborist has to review the proposal and show whether the tree can still be kept viable. If you get a report that classes an encroachment as moderate and leaves it there, that assessment has not been done and council can ask for it.
5. Reporting is staged now
The single catch all Tree Management Plan has been replaced by a sequence, each with a defined job:
PAR, Preliminary Arboricultural Report
Done before the design is drawn. Records what is on the site, which trees are protected and where the root zones fall, so the architect knows the constraints early.
AIA, Arboricultural Impact Assessment
The document that goes in with the permit application. Sets out what the proposal does to each tree, with the zones calculated and the encroachment measured and justified.
TPP, Tree Protection Plan
A marked up site plan showing which trees are being protected, where the fencing goes and what protection zone has been applied to each one.
TPS, Tree Protection Specifications
The written conditions that go with the plan. What may happen inside the zone, the hold points, how any excavation near roots is carried out, and who signs off each stage.
Most Victorian councils still ask for a "Tree Protection Management Plan" on their checklists, and several Melbourne consultancies still sell one. The practical answer is to give them a document that satisfies the request while being structured to the current standard, and to say so in a line at the front.
6. What to do when a council checklist is written in 2009 language
You will meet this constantly, because councils update at their own pace. Some concrete examples from the councils I work in:
- Banyule states that AS 4970-2009 is the primary guidance for preparing arboricultural reports, and hosts a copy of the 2009 standard on its own website as a reference document
- Merri-bek's tree protection technical note, dated September 2022, calls AS 4970-2009 the "current" standard
- Boroondara sets its measuring point at 1.4 m "as set in the Australian Standards 4970-2009"
- Council Arboriculture Victoria has moved to a 2025 calculator, but plenty of council documents still link to it as though it were the old one, so check which version you are actually on
None of that makes the 2009 edition current. What it does mean is that if you hand a council a report it cannot line up against its own checklist, you are likely to get a request for further information back.
What I do is report in the current terminology and put the old equivalent in brackets the first time each term appears. "Notional Root Zone (NRZ), previously the calculated TPZ under AS 4970-2009." It costs one line in the report and it stops the planner having to email and ask.
If you are buying the standard, check the edition
AS 4970:2025 is a second edition and it was published with Amendment No 1 incorporated in December 2025. A copy bought earlier in 2025 will not have it. Check the front matter of whatever you are working from, and if you are being handed a report, it is fair to ask which edition the author used.
On this article
This is a working summary written to help you understand what changed, not a substitute for the standard itself. If you are preparing reports, buy the standard and read it. If you are commissioning one, this should be enough to tell whether the report you receive was written to the current edition or to an old template.
Common questions
Is AS 4970-2009 still valid?
No. It has been superseded by AS 4970:2025. Plenty of council guidance still references the 2009 edition, and that is a currency problem with the guidance rather than a reason to report to a superseded standard.
Did the TPZ calculation change?
The multiplier did not. NRZ radius is still trunk diameter multiplied by twelve, with the same 2 m and 15 m clamps. What changed is what the resulting number is called and what it means: it is now the notional zone, and the TPZ is what the project arborist actually applies.
What is the difference between NRZ and SRZ?
NRZ is the broad rooting area, calculated from DSH, and it is about the tree's resources. SRZ is the smaller zone around the base that holds the tree upright, and it is about stability. Any encroachment into the SRZ is classed as major.
How do I measure a leaning or multi stemmed tree?
Multi stem is the square root of the sum of the squares of the stem diameters. For a leaning tree, run the tape 1.4 m along the stem rather than 1.4 m up from the ground, and measure square to the trunk. Say in the report that this is what you did, because the figure will not match one taken vertically.
Working on a site this applies to?
Send the address and whatever plans you have. Fixed quote back within one business day, and if it turns out you do not need a report I will tell you that instead.