What councils actually check in an impact assessment

1. Who the report is actually for

The person reading your assessment is usually a planner, not an arborist. They may have a council arborist to refer to, and on a contentious site they will, but the first read is by someone assessing an application against a set of controls.

That changes what a good report looks like. A planner is not evaluating your diagnostic reasoning. They are checking whether the application can be assessed on what you have given them. If they cannot answer their own questions from your document, it goes back, regardless of how sound the arboriculture underneath it is.

Most reports that get sent back are not wrong. They are unanswerable.

2. What has to be in it

Requirements vary between councils and between states, and your own council's checklist always wins over any general list. That said, the following turns up almost everywhere:

• Every tree identified individually. Species, age class, origin, a tree number that matches the plan, and dimensions including DSH.
• Health and structure for each tree, with the defects that matter to the recommendation actually named rather than summarised as a rating.
• NRZ and SRZ figures calculated and stated, and the TPZ you have determined for the site.
• Impact analysis against the actual proposal. Not a description of the trees, but what the works do to each one.
• Neighbouring trees whose zones reach into the site. Leaving these out is one of the most common gaps of all, and one of the easiest to avoid.
• Tree protection measures that are specific: where fencing goes, what triggers supervision, what happens within the zone.
• A clear recommendation per tree, retain or remove, with the reasoning attached to it.
• A plan showing tree locations and zones over the proposed works.
• Photographs tied to the trees they show.

3. The five gaps that trigger a request for further information

Describing the trees without assessing the impact

This is the big one. A thorough inventory that never quite says what the proposed building does to tree 7 leaves the planner with nothing to assess. Every retained tree needs a sentence connecting the works to that tree and stating the consequence. Where the encroachment is minor, say so and say why. The absence of a statement reads as an oversight, not as a nil impact.

Neighbouring trees left out

A tree over the fence whose root zone reaches into the works is squarely within scope, and the neighbour is often the objector. Leaving it out looks like either an oversight or an omission, and neither is a good look in a planning response.

Measurements that do not add up

A DSH in the schedule that does not match the zone in the table, or a tree numbered differently on the plan and in the text. It reads as carelessness, and it makes a planner wonder what else is wrong. This is exactly the kind of error that creeps in when data is written on a form on site and typed up hours later, which is a whole topic of its own.

Protection measures written as generic notes

"Standard tree protection measures to be implemented in accordance with AS 4970" tells the planner nothing they can condition. Where fencing goes, at what radius, when it goes up, who supervises, and what may and may not happen inside it. Specific enough to be enforced is the bar.

Not addressing the actual overlay

If the site sits under a vegetation protection or significant landscape overlay, the report should speak to it directly. A general assessment that never mentions the control it exists to satisfy invites a request asking you to do exactly that.

4. The qualification question

Most Victorian councils require reports from an arborist holding a minimum of AQF Level 5, the Diploma of Arboriculture. Other states and individual councils vary, and some also want to see relevant risk assessment credentials on reports dealing with tree failure.

Two practical points. Put your qualifications in the report, on the front page or in the signature block, because a planner should not have to go looking. And if you are a client choosing an arborist, ask before you engage rather than after the report is rejected, because a report from an underqualified author can be refused on that basis alone no matter how good the content is.

5. A 2025 trap worth avoiding

AS 4970 was revised in 2025 and the terminology moved. What used to be the calculated TPZ is now the NRZ, and TPZ now means the area you actually protect on the ground. DBH became DSH.

Councils are working through the same transition, at different speeds. You will get checklists and requests written in 2009 language for a while yet. Two things help:

• Report in current terminology, and add one line explaining the equivalence the first time each term appears. It takes a sentence and it prevents the confusion rather than answering it later.
• State both the NRZ and the TPZ you have determined, as separate figures. Under the new structure they are different things, and where you have set a protection distance different from the calculated one, that is a professional judgement worth writing down rather than hiding.

The full rundown of what changed is written up separately, and there is a free calculator if you want to check figures.