Encroachment is not impact: when existing conditions change the assessment of works near trees

The native vegetation guidelines deem a tree lost once works encroach more than 10 per cent into its notional root zone, unless an arborist report indicates otherwise. In June 2026 the Priority Projects Standing Advisory Committee accepted that 193 trees along two road reserves at Grantville could be retained, although most had more than 20 per cent encroachment. The assessment measured impact, not encroachment, and that distinction is the subject of this article.

Encroachment is a measure of area. Impact is the effect of the works on the roots that are actually present. On ground that has been a road for decades, the two are not the same.

The matter in brief

Road upgrade works along Stanley Road and McGrady Road, Grantville, required the removal of native vegetation from two road reserves through remnant Lowland Forest. Bass Coast Shire Council refused the permit under Clause 52.17 and the Significant Landscape Overlay. The refusal went to VCAT, and in September 2025 the Minister for Planning called the matter in and referred it to the Priority Projects Standing Advisory Committee. The Committee heard the matter in May 2026, reported on 17 June 2026, and recommended that a permit issue. The permit has since been granted. Arbor Survey gave the arboricultural evidence.

The deeming rule

Under the native vegetation framework, the extent of removal includes assumed losses as well as trees that are felled. The Assessor's Handbook lists three sources of assumed loss: excessive lopping and the proximity of the use or development to trees, native vegetation around dwellings, and indirect impacts. For proximity, it notes that compaction and excavation close to tree roots cause indirect losses, points to AS 4970:2025 for the notional root zone and structural root zone, and sets this default:

"Unless an arborist report indicates otherwise, a tree, or trees will be deemed lost if the encroachment (of compaction and excavation) into the NRZ is greater than 10 per cent, or is inside the SRZ."

The notional root zone is the AS 4970:2025 calculation: a radius of twelve times the trunk diameter at 1.4 metres, with a minimum of 2 metres and a maximum of 15 metres. The structural root zone is the smaller area needed for stability. Both are circles drawn from the trunk. They say nothing about where the roots are.

The rule is the default where there is no arborist report. The qualifier, "unless an arborist report indicates otherwise", is the whole point. Where an arborist has assessed the trees, the assessment governs. The Handbook applies the same logic to lopping: lopping beyond the Clause 52.17 maintenance exemption is treated as loss of the tree "unless an arborist report concludes that the tree will survive".

Why encroachment and impact diverge on a road corridor

Encroachment is the share of the notional root zone that the works occupy. It is geometry. Impact is what the works do to the roots that are actually present, which depends on where those roots are.

On an established road the sealed pavement and the table drains have sat inside the notional root zone of every roadside tree for decades. The ground under the pavement is compacted and sealed. The drain inverts are cut below the natural surface. Root occupation under that ground is reduced, and in places absent, because the conditions that would support roots have not been there for a long time. AS 4970:2025 recognises this in the way the tree protection zone is set: the project arborist starts from the notional root zone and then takes into account the matters in cl. 3.3.2, which include existing obstructions and hard surfaces, before deciding the zone that is protected on the ground.

The consequence is that two trees with the same encroachment figure can face very different impacts. A tree in open ground with 30 per cent of its notional root zone excavated loses roots it is using. A roadside tree with 30 per cent of its notional root zone inside a works footprint that stays within the existing pavement and drain inverts may lose very little, because there is little there to lose. The encroachment figure is the same. The impact is not.

How the assessment was done

The assessment put before the Committee treated the existing road as the baseline and assessed each tree on its own.

The road cross-sections from the final design were laid over the tree survey in CAD. For each tree, the new excavation was compared with the existing drain inverts, and the outer edge of the batter was taken as the limit of works. Trees inside the works footprint were recorded as lost and counted in full for offsets, including 11 that may in fact survive. Trees outside the footprint were placed in an encroachment band under AS 4970:2025, major (over 20 per cent of the notional root zone, or inside the structural root zone), moderate (over 10 to 20 per cent) or minor (10 per cent or less), and then assessed for impact. A tree was shown as retainable where the works, in most cases, stayed within the ground the road already occupied between the drain inverts.

Retention was conditional. Every retained tree was tied to a protection regime under AS 4970:2025: a tree protection specification and plan, a project arborist on site, supervision and certification at set stages, specifications for any fill over root zones, and a watering regime.

The objection, and what the Committee found

Council's objection went to the method. Of the 193 trees shown as retained, 146 had 20 per cent or more encroachment into the notional root zone, most at 35 to 40 per cent. No roots had been exposed and mapped. On that basis Council submitted that the true loss was unknown and the assessment was inconsistent with the Handbook.

The Committee did not accept that submission. It found that the arboricultural assessment was an appropriate expert assessment of the impacts of the works on the trees, accepted the opinion that the trees shown as retainable could be retained despite encroachment above 10 per cent, and stated that the assessment was consistent with industry standards and relevant guidance.

The Committee accepted that there was "some uncertainty regarding the long term viability of 'retained' trees especially those that will experience greater than 20 per cent encroachment", and dealt with it through the permit conditions rather than by discounting the evidence. Those conditions require a Tree Management Plan before works begin: a tree protection specification and plan to AS 4970:2025, a project arborist engaged to the satisfaction of the responsible authority and present during the works, written certification before works start and at intervals during them, a 48 hour reporting rule for any departure that affects a retained tree, a watering regime for at least 12 months after completion, and specifications for the make-up and depth of fill over root zones.

What this means for an arborist report

The deeming rule leaves room for an assessment of actual impact. The room is only there for an assessment that can be tested. The elements that held up at Grantville were a record for each tree, a stated limit of works, a stated basis for treating the existing disturbance as the baseline, a clear separation of trees counted as lost from trees assessed as retainable, and a protection regime attached to every retained tree. A report that asserted survival without that record would not displace the rule, and should not.

The same reasoning applies wherever hard surfaces have occupied part of a root zone for a long time: road reserves, rail corridors, pipeline easements, the edges of older subdivisions. In each case the encroachment figure is where the assessment starts, not where it ends.