Reports Explained
AS 4970:2025 — What Changed, and What It Means for Your Project
AS 4970:2025 superseded the 2009 standard on 19 December 2025. The Tree Protection Zone became the Notional Root Zone, DBH became DSH, and encroachment went from one threshold to three bands. Here is what actually changed and what it means if you are building near a tree.
AS 4970:2025 superseded AS 4970-2009 on 19 December 2025. If you are holding an arboricultural report that cites the 2009 standard, it is citing a superseded document. That does not automatically make its conclusions wrong — the core arithmetic is unchanged — but the terminology it uses is out of date, and one of its judgements may now fall in a different category.
Working numbers? The tree protection zone calculator is updated for the 2025 standard, including the three new encroachment bands.
The short version
| AS 4970-2009 | AS 4970:2025 | |
|---|---|---|
| Theoretical root area | TPZ = DBH × 12 | NRZ = DSH × 12 |
| What “TPZ” means | the theoretical zone | the physical area fenced on site |
| Trunk measurement | DBH, diameter at breast height | DSH, diameter at standard height |
| Measurement point | 1.4 m above ground | 1.4 m above ground — unchanged |
| SRZ formula | (D × 50)0.42 × 0.64 | unchanged |
| Encroachment | one threshold: >10% is major | three bands — see below |
| SRZ encroachment | assessed case by case | any intrusion is automatically major |
| Documents | Tree Protection Management Plan | TPP (drawing) + TPS (written specs) |
The arithmetic did not change. The names did, the thresholds did, and the documents split in two.
The Notional Root Zone
The area that used to be called the Tree Protection Zone is now the Notional Root Zone (NRZ). It is still trunk diameter multiplied by twelve, still capped at a 15 metre radius, and still the theoretical area a tree needs to stay healthy.
The rename exists because “Tree Protection Zone” was doing two jobs at once. It meant both the calculated area on a drawing and the fenced-off area on a building site, and those are rarely the same shape — site constraints, driveways and existing structures mean the fencing goes where it can go, not where a perfect circle would put it.
So the 2025 standard splits them. NRZ is the calculation. TPZ is now specifically the physical area you actually protect. On a real site the TPZ is often smaller than the NRZ, and the report’s job is to justify the difference.
DBH became DSH
Diameter at Breast Height is now Diameter at Standard Height. The measurement point is unchanged at 1.4 metres above ground, so every DBH figure you already have is a DSH figure — no remeasuring required. Only the name moved.
Encroachment: one threshold became three
This is the change most likely to affect a project already in design.
Under the 2009 standard there was a single boundary. Encroachment above 10% of the protection zone was major, below it was minor, and that was the whole framework.
AS 4970:2025 introduces a middle band:
- Minor — up to 10% of the NRZ, outside the SRZ. Standard protection measures are normally enough.
- Moderate — between 10% and 20% of the NRZ, outside the SRZ. Requires review by a project arborist. This band is new.
- Major — more than 20% of the NRZ, or any encroachment at all into the SRZ. The project arborist must demonstrate the tree will remain viable, which in practice means root investigation before the design is fixed.
The practical effect: a project taking 15% of the root zone was major under the old standard and is moderate under the new one. That is a genuinely lower bar — review rather than full justification — and it is the single most useful thing in the revision for anyone designing around a retained tree.
Working the other way, any intrusion into the Structural Root Zone is now automatically major, however small. There is no such thing as a minor encroachment into the roots holding a tree upright, and the standard now says so explicitly.
TPMP became TPP and TPS
The Tree Protection Management Plan split into two documents:
- Tree Protection Plan (TPP) — the drawing. Where the fencing goes, what is protected, what the site team looks at.
- Tree Protection Specifications (TPS) — the written instructions. Ground protection, permitted works inside the zone, supervision requirements, hold points.
Referring to a “TPMP” is still acceptable if you make clear it contains both. In practice the split is an improvement, because the drawing is what goes on the site fence and the specification is what goes in the contract, and they were always read by different people.
What this means for you
If you have a report citing AS 4970-2009, it is not void. The NRZ figure in it is the same number the 2025 standard produces. But it is worth checking two things: whether an encroachment it called “major” now falls in the moderate band, and whether the council assessing it expects current references.
If you are commissioning a report now, it should cite AS 4970:2025 and use the current terminology. A report issued in 2026 against a superseded standard is the kind of thing an assessing officer notices.
If you are in design, the moderate band is worth knowing about before you finalise setbacks. A layout that was previously pushed out to stay under 10% may now have more room than you were told.
Standards Australia published AS 4970:2025 on 19 December 2025 as the second edition, superseding AS 4970-2009. The standard itself is available from Standards Australia; this guide summarises the changes rather than reproducing it.
This guide is general information, not legal advice. Council rules change — always confirm current requirements with the relevant council before undertaking tree work.