Why AS 4970-2025 matters for your DA
If you’re designing, lodging or building anything on a NSW site with trees, AS 4970-2025 — the Australian Standard Protection of Trees on Development Sites — sets the rules for how those trees are assessed, protected and (where unavoidable) impacted. Most NSW council Development Control Plans (DCPs) reference the standard either directly or by adoption. Your Arboricultural Impact Assessment will be measured against it. Your construction site will be inspected against it.
AS 4970 was last meaningfully updated in 2009. The 2025 edition is the first significant revision in 16 years and changes some terminology that matters for how reports read and how the design conversation happens on site.
In this post
What actually changed in 2025
The 2025 edition is best understood as a terminology clarification plus a cleaner encroachment framework. The underlying geometry of how you protect a tree on a development site hasn’t changed. The way you name the components has.
Three substantive changes:
- The “TPZ” is now the physical fenced area on site, not the radius calculation. The calculation that defines its dimensions is called the Nominal Root Zone (NRZ).
- Encroachment is now classified in three tiers: Minor, Moderate, Major. The old binary “acceptable / unacceptable” framing has been replaced.
- Mitigation requirements are more specifically tied to the encroachment tier, and air spade investigation is mandated in Moderate and Major encroachments (used to be discretionary).
Smaller wording changes throughout, but the headline is the NRZ / TPZ rename and the three-tier encroachment system.
The NRZ vs TPZ rename, in plain English
This is the change that most often trips up architects and builders who learned AS 4970-2009 and are now reading 2025-edition reports.
| AS 4970-2009 (old) | AS 4970-2025 (current) | |
|---|---|---|
| The calculation | “TPZ” (formula: DBH × 12) | “NRZ” (Nominal Root Zone) — same formula |
| The physical fenced area on site | Often called “TPZ fencing” | “TPZ” (Tree Protection Zone) — the actual fenced area |
| What goes on the site plan | “TPZ shown as circle around tree” | “NRZ shown as circle, TPZ fencing shown as installed boundary” |
Put another way: the 2025 edition is just being more precise. The calculation that tells you how big the protection zone needs to be is now called the NRZ — the physical fence on site that implements that protection is what gets called the TPZ. They’re often the same shape geometrically (a circle of radius NRZ around the tree), but in 2025 they have distinct names so reports can talk about both without confusion.
If your council DCP or older AIA template still says “TPZ” when it means the calculation, that’s fine — everyone reading it will understand. Just don’t be surprised when a 2025-aligned arborist report uses “NRZ” for the radius and “TPZ” for the fenced area.
Need to calculate the NRZ / TPZ for your trees?
Free online tool. Enter trunk diameter, get the NRZ radius, SRZ radius and 3-tier encroachment classification per AS 4970-2025. Multi-zone, multi-stem, with printable site plan.
SRZ — what hasn’t changed
The Structural Root Zone (SRZ) is the smaller zone within the NRZ containing the woody buttress roots that physically hold the tree up. The formula is unchanged from 2009:
SRZ radius = ((D × 50)^0.42) × 0.64 — where D is the trunk diameter at ground level, in metres.
The SRZ is typically 1.5–4 metres for a typical established tree. Encroachment inside the SRZ is treated as Major impact under AS 4970-2025 regardless of percentage — cutting into the buttress roots is what destabilises trees.
Encroachment classification: the 3-tier system
This is the practical change most architects and engineers feel in their day-to-day work. AS 4970-2025 classifies impact on each retained tree by the percentage of the NRZ being encroached:
| Tier | NRZ encroachment | Treatment |
|---|---|---|
| Minor | ≤ 10% | Generally acceptable with standard tree protection. No air spade investigation required. |
| Moderate | > 10% to 20% | Air spade investigation required to confirm root distribution. Specific mitigation protocols apply. |
| Major | > 20%, OR any SRZ encroachment | Air spade investigation mandatory. Major mitigations required. May trigger design reconsideration or removal. |
The three-tier approach matters because it gives designers and councils a common shared language. “Moderate encroachment” means the same thing in your AIA, your DA assessment and your council pre-DA discussion.
In practice, councils now want each retained tree explicitly classified Minor / Moderate / Major in the AIA, with mitigation appropriate to the tier.
Mitigation hierarchy for each tier
AS 4970-2025 sets out mitigation expectations that scale with the tier:
Minor encroachment (≤ 10% NRZ)
- Standard TPZ fencing (1.8 m chain-wire on steel star pickets at max 2 m centres)
- Ground protection where the unavoidable encroachment occurs (porous geotextile + 100 mm hardwood mulch, or engineered panels)
- Hand or air spade excavation for any subgrade works in the encroachment area
- Project Arborist sign-off at pre-construction inspection
Moderate encroachment (> 10–20% NRZ)
- All of the above, plus:
- Air spade root investigation in the encroachment zone, mapping the actual root architecture
- Specific root pruning protocol with clean hand cuts for any root over 50 mm diameter
- Arborist sign-off on each cut root over 50 mm
- Increased supervision frequency during the encroachment works
Major encroachment (> 20% NRZ, or any SRZ encroachment)
- All of the above, plus:
- Design reconsideration is the first option — can the encroachment be reduced?
- If not, comprehensive air spade investigation + biomechanical assessment of the tree’s future stability
- Potentially additional structural support (cabling, bracing) post-works
- Where Major encroachment cannot be mitigated to bring risk to acceptable levels, council may require removal with replacement plantings to compensate
The practical implication. A design that encroaches “a little” into a TPZ used to be acceptable under the old binary system as long as you put up fencing. Under AS 4970-2025 that same encroachment may now sit in the Moderate tier and trigger mandatory air spade investigation. Council assessment is more granular — and the cost of mitigation is correspondingly higher.
What this means for design and construction
The 2025 edition rewards better design earlier. Three practical takeaways:
1. Run the NRZ calculation before you finalise the footprint
If you can shift a slab edge or driveway by half a metre to drop a tree from Moderate to Minor encroachment, you’ve just saved an air spade investigation ($1,500–$3,000 per tree) and bought yourself a smoother DA assessment.
2. Commission an AIA at the right moment
An AIA done before the design is locked in is partially wasted work — if the design shifts, the AIA needs to be redone. An AIA done with a finalised design captures the actual encroachment per tree under AS 4970-2025 once and accurately.
A Preliminary Arboricultural Report (PAR) at concept stage, followed by an AIA at DA stage, is usually the cost-effective path for any site with multiple trees.
3. Budget for air spade investigation
If your design has Moderate or Major encroachments, mandatory air spade investigation needs to be budgeted — both in cost and in time (it adds a week or two to the DA timeline). For sites with several encroachments, in-house air spade capability (as opposed to outsourcing) significantly reduces the lead time.
Not sure if you need a PAR or an AIA?
Free 5-question wizard tells you which arborist report your project needs, with indicative cost and turnaround for each.
Use the AS 4970-2025 calculator
The fastest way to translate a design into AS 4970-2025 numbers is our free TPZ / NRZ / SRZ + Encroachment Calculator. Enter your tree’s DBH, sketch in the encroachment zones, and the tool gives you:
- NRZ radius (the protection-zone calculation)
- SRZ radius (the structural-root zone)
- Encroachment percentage classified Minor / Moderate / Major per AS 4970-2025
- Whether air spade investigation is triggered
- A printable site plan with all zones drawn to scale
It handles multi-stem trees (combined DBH), multiple encroachment zones (correctly unioned, not double-counted), and per-zone angle so the diagram matches your actual site layout. Indicative only — doesn’t replace a certified AIA, but it gets you the numbers fast.
FAQ
Is AS 4970-2025 mandatory in NSW?
Not by direct legal force, but practically yes. Most NSW council DCPs either reference the standard directly or have adopted equivalent protective provisions. A DA with retained trees is normally expected to comply with AS 4970, and an AIA that doesn’t reference the current edition will likely attract an RFI from council. Expert evidence at NSW LEC also tracks AS 4970.
Do I need to re-issue an old AIA written against AS 4970-2009?
Usually no, if the AIA was approved with your DA and the project is proceeding. For a new DA, you should be using AS 4970-2025. For an RFI on an existing matter, follow what council asks. Existing approved DAs with AS 4970-2009 AIAs continue to operate under the conditions of consent.
What’s the practical difference between Moderate and Major encroachment?
Moderate (>10-20%) triggers air spade investigation and clean root-pruning protocols, but the design is usually workable. Major (>20% or any SRZ) tends to trigger a design conversation: can the building shift? If not, comprehensive investigation + potential structural support post-works + replacement-planting condition is likely. Major encroachment on a high-value tree often results in council pushing back on the design.
What’s the cost of an AIA per AS 4970-2025?
Assurance Trees writes AS 4970-2025 compliant AIAs from $1,800 + GST. Single-tree assessments quoted separately. Sites with 11+ trees or significant trees attract higher rates — final scope confirmed after site review. Use our PAR vs AIA Decision Tool for indicative cost banding for your specific project.
Where can I read AS 4970-2025 itself?
AS 4970-2025 is published by Standards Australia. The full text is available for purchase from store.standards.org.au — it’s not free. Most consulting arborists hold a current licensed copy; reports cite the standard rather than reproducing it.
Note. This post is plain-English educational content, not a substitute for AS 4970-2025 itself or for site-specific arboricultural advice. For your project, engage a qualified consulting arborist for a written AIA. Assurance Trees Pty Ltd accepts no liability for outcomes arising from reliance on this educational material.
Need an AS 4970-2025 compliant AIA?
Assurance Trees writes council-ready Arboricultural Impact Assessments to AS 4970-2025 across NSW. AQF Level 5 Consulting Arborists, NSW Licensed Builder, in-house air spade, 15+ years of practice. From $1,800 + GST.
See AIA service Free tools