AS 4970:2025: what changed, and what your council now asks for
The standard was revised in 2025 and the vocabulary moved underneath everyone. Six of the eight best known consultancies in Melbourne still do not mention it. Two still advertise the 2009 edition. Council Arboriculture Victoria's public calculator is still built on the old formulas.
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 number, one name, one job.
The 2025 revision splits that in two:
They are frequently the same figure. They are not the same idea, and on a constrained site they are often different numbers.
The calculated zone is now a starting point for professional judgement, not the answer.
That has a practical consequence for reports. Stating only one figure is no longer enough. A report should give the NRZ, then state the TPZ determined for the site, and where the two differ it should say why. That difference is the professional judgement you are paying for, and writing it down is what makes it defensible.
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 measured at 1.5 m until 1 September 2026 and now measures at 1.4 m.
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 much smaller than the NRZ and it matters much more. Encroaching on it is not a proportional loss of rooting area, it is a stability question.
Multi stemmed trees
Combined DSH is the square root of the sum of the squares of the individual stem diameters:
DSHcombined = √(D1² + D2² + D3² …)
Not the sum of the diameters, which overstates it badly, and not the largest stem, which understates it. Three 300 mm stems give a combined DSH of about 520 mm, not 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 note. First, area, not radius. People routinely calculate the encroachment as a proportion of the radius, which understates it dramatically, because area scales with the square. Second, any intrusion into the SRZ is major regardless of how small the NRZ encroachment is. There is no minor SRZ encroachment.
The moderate band now carries an obligation
Under the 2025 standard, moderate encroachment is not simply a category you report. The project arborist is expected to review the proposal and demonstrate whether the tree's viability can be maintained. A report that says "moderate encroachment" and stops has not finished the job the standard sets 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
Early stage. What is on site, what is protected, what will constrain the design. Before the lines are drawn.
AIA, Arboricultural Impact Assessment
The permit document. What the proposal does to each tree, zones calculated, encroachment quantified and justified.
TPP, Tree Protection Plan
The drawing. Which trees are protected, where the fence goes, what the applied TPZ is for each.
TPS, Tree Protection Specifications
The words. What happens inside the zone, hold points, excavation methodology, who supervises what.
Most Victorian councils still ask for a "TPMP" 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, the reference body many Victorian councils defer to, still publishes a 2009 calculator
None of that makes the 2009 edition current. It does mean handing a council a document it cannot map onto its own checklist is a good way to buy a request for further information.
What works: report in current terminology, and the first time each term appears, add one line giving the equivalence. "Notional Root Zone (NRZ), previously the calculated TPZ under AS 4970-2009." It takes a sentence and it prevents the confusion instead of answering it six weeks later.
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, measure perpendicular to the trunk axis at 1.4 m along the stem rather than 1.4 m vertically, and say in the report which you did.
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.