AS 4970:2025: what actually changed
The standard we all work to was rewritten last year, and it renamed several of the things you calculate every single day. Most of the maths survived intact. The words did not. Here is what moved, why it moved, and what you need to change in your templates before your next report goes out.
What is covered
AS 4970:2025 superseded the 2009 version on 30 May 2025. If you have been in this industry any length of time, you learned the old vocabulary well enough that it comes out automatically when a builder asks you a question on site. That is exactly why this revision is worth twenty minutes of your attention, because the trap is not misunderstanding the new standard. The trap is using the old words correctly out of habit and having them mean something different now.
1. The TPZ was doing two jobs, and now it does one
This is the change that matters most, and it fixes something that caused genuine arguments in planning responses for fifteen years.
Under the 2009 standard, TPZ meant both the number that came out of the formula and the area you fenced off on site. In practice everyone used it to mean whichever they needed at the time. A council officer would read "TPZ 14.4 m" in a report and treat it as a fixed boundary. The arborist writing it meant it as a starting point that site conditions might reasonably move. Both were using the same term. Neither was wrong. That is a bad way to run a standard.
The 2025 revision splits it in two:
| Term | What it means now | Where it comes from |
|---|---|---|
| NRZ | Notional Root Zone. The theoretical zone that comes out of the formula. This is precisely what the 2009 standard called the TPZ. | DSH (mm) × 0.012min 2 m, max 15 m |
| TPZ | Tree Protection Zone. The area actually protected on the ground, which the project arborist determines. | Your professional judgement, informed by the NRZ |
| SRZ | Structural Root Zone. Unchanged in meaning and in formula. | ((D × 50) ^ 0.42) × 0.64min 1.5 m |
The formula gives you the NRZ. You give the site its TPZ. The standard has finally written down what good practitioners were doing anyway.
In practical terms this is a gift, because it makes your professional reasoning legible. When you set a protection distance larger than the calculated figure because the tree is a mature Corymbia in compacted clay with a history of trenching down one side, you are no longer contradicting the number in your own report. You are doing the job the standard describes: taking the notional figure and determining an actual zone.
It also means a sentence that used to be awkward is now simply correct. "The NRZ is 14.4 m. The TPZ has been set at 16 m for the reasons below." Under the old wording that read like an error. Under the new wording it reads like an arborist doing their job.
2. DBH is now DSH
Diameter at Breast Height has become Diameter at Standard Height. The measurement itself is identical, still taken at 1.4 m above ground level.
The reason is straightforward: breast height was never a defined dimension. It is an inherited convention from forestry, and it varies by country. Standard Height is a defined number, which is what a standard ought to contain.
Nothing about your field practice changes here, though where you put the tape still catches people out. Your old DBH measurements remain valid and directly usable. What changes is the label in your report templates and your data fields, and it is worth doing rather than leaving, because a report that says DBH in 2026 quietly signals that its author has not read the current standard.
3. Encroachment has three defined tiers
The old standard handled encroachment loosely. Ten percent was the widely used line for a minor encroachment, with a fair bit of interpretation on either side of it and a lot of variation between councils.
AS 4970:2025 sets out three tiers, measured as a percentage of the NRZ area:
| Tier | Threshold | What it means for the job |
|---|---|---|
| Minor | 10% or less of NRZ area, clear of the SRZ | Generally acceptable with standard protection measures, still documented and reasoned in the report. |
| Moderate | More than 10% and up to 20%, clear of the SRZ | Expects review by the project arborist, usually supported by root investigation, with specific mitigation rather than generic protection notes. |
| Major | More than 20%, or any SRZ encroachment at all | A significant impact. May not be supportable. Where the tree is retained you need root investigation and a properly argued case. |
Two things people get wrong here.
First, encroachment is measured by area, not radius. This catches people out constantly, and it catches them out in the direction of being unnecessarily alarmed. An incursion that looks substantial on a plan because it reaches some way in from the edge is often well under ten percent of the circle's area, because the outer band of a circle holds far less area than it appears to.
Second, any SRZ encroachment is major, full stop. There is no percentage calculation to fall back on and no arguing it down to moderate. If the works enter the structural root zone, that is the classification.
If you want to check a figure quickly, we built a free NRZ and SRZ calculator that does the area maths and classifies the tier for you. It runs entirely in your browser, so it works on site with no reception.
4. What did not change
Worth saying plainly, because the terminology shift makes it feel like more moved than actually did:
- The NRZ calculation is the same maths as the old TPZ calculation. Twelve times the trunk diameter, 2 m minimum, 15 m maximum.
- The SRZ formula is untouched.
- Where you measure, and how, is the same.
- Your existing measurements remain valid. Nothing needs remeasuring.
So if you took a tree's dimensions in 2023, the numbers you calculated then are the numbers you would calculate today. Only their names have changed.
5. What to fix in your templates
A short list you can work through in an afternoon:
- Rename the calculated field in your report templates and tree schedules from TPZ to NRZ.
- Add a separate TPZ field for the zone you have actually determined, so the difference is visible where it matters.
- Change DBH to DSH everywhere, including field sheets and spreadsheets.
- Update your encroachment wording to the three tiers, and state which one applies rather than describing the impact in prose.
- Update the standard reference in your citations from AS 4970-2009 to AS 4970:2025.
- Where you set a TPZ different from the NRZ, write the reasoning down. The new structure invites this, and it is your best protection if the report is ever questioned.
The councils you deal with are working through the same transition, and not all of them are through it. It is worth knowing what they check in the report itself too. Expect a period where you receive requests worded in 2009 language. Answering in 2025 terms, with a line explaining the equivalence, tends to move things along faster than quietly matching their vocabulary.
On this article. This is a practitioner's summary written to be useful, not a substitute for the standard itself. AS 4970:2025 is published by Standards Australia and if you are producing reports against it you should be working from your own copy. If you spot something here that does not match the standard, tell us at hello@timbrfield.com.au and we will correct it the same day and say that we did.
The app that keeps up with the standard
TIMBR Field records tree assessments on site, works out the NRZ and SRZ as you enter the measurements, tags your photos with GPS, and syncs everything into your own Microsoft 365 tenant. It is in private beta with a small group of Australian consultancies, and there are five places left in the current intake.