AS 4970 tree protection on a Cardinia construction site.
You’re keeping the big Manna Gum in the corner of the block. The planning permit says you must. AS 4970-2009 sets out how: calculate the TPZ, fence it, no-dig inside it, water it through the build, and sign it off at four milestones. We write the report, run the fencing inspection, and certify compliance.
The four pillars of AS 4970-2009 compliance.
AS 4970-2009 “Protection of trees on development sites” is the Australian Standard that Cardinia Shire planning permits point to when they say “the existing tree shall be retained and protected throughout construction.” The standard is short, sensible and almost always misunderstood by builders the first time they encounter it. Four pillars.
Pillar 1. Calculate the TPZ.
The Tree Protection Zone is the circular area around the trunk inside which the tree’s feeder roots and structural roots live. Formula: TPZ radius = 12 × Diameter at Breast Height (DBH, measured at 1.4m). Minimum 2m, maximum 15m. For a 60cm DBH mature River Red Gum that’s a 7.2m radius circle — a 14.4m diameter no-build envelope. We mark the TPZ on the site plan in red, and we mark the Structural Root Zone (a smaller inner circle using a separate formula) in orange. Inside the SRZ is genuinely no-go — cutting structural roots can drop the tree.
Pillar 2. Fence the TPZ.
1.8m steel mesh panels with concrete feet, signed at every visible face, up before site establishment, down only at practical completion. Mulch the ground inside the TPZ to 100mm before fencing to reduce compaction risk. Bunting and star pickets — the domestic shortcut — is not AS 4970 compliant and will get flagged by Council’s building surveyor on a routine inspection.
Pillar 3. No-dig inside the TPZ.
No excavation, no soil cut, no soil fill, no stockpiling of materials, no vehicle traffic, no concrete or chemical washout, no temporary site office, no portable toilet. If a service trench genuinely must enter the TPZ — sewer connection, stormwater, power conduit — AS 4970 allows it under three conditions: hand-dig only, supervised by the Project Arborist, with any root over 25mm preserved and re-buried (not cut). Air-spade excavation is the gold-standard method for these incursions and we carry one on the truck.
Pillar 4. Water it through the build.
Construction is a drought event for the kept tree. Compaction reduces water infiltration even outside the TPZ. Runoff patterns change. Some feeder roots will be lost despite best efforts. The watering regime — deep-soak monthly cool season, fortnightly warm season — keeps the tree in stress range A rather than stress range C.
Why Cardinia AS 4970 jobs need extra care.
A few patterns we see repeatedly on Cardinia Shire AS 4970 work, worth flagging up front.
Indigenous-remnant species are usually the keepers. On growth-corridor subdivisions (Pakenham, Officer, Cardinia Lakes, Heritage Springs) the trees retained on individual lots are almost always the surviving River Red Gum, Yellow Box or Manna Gum that pre-date subdivision. These have been there since before the bulldozers. The root systems extend well beyond the obvious TPZ and have already adapted to the original landform — which means soil cuts and fills around them are particularly damaging.
BAL bushfire overlay interaction. In the hills district (Beaconsfield Upper, Cockatoo, Emerald, Gembrook, parts of Avonsleigh) most building lots are in a Bushfire Management Overlay and have a BAL rating — commonly BAL-29 or BAL-40. Defendable space requirements can conflict with AS 4970 TPZ rules — the bushfire engineer wants the canopy reduced or the tree gone, the planning condition wants it kept. We work this conflict out with the bushfire engineer and the consulting arborist at planning stage so it doesn’t turn into a stop-work order at frame stage.
Builder education up front. Most builders have done one or two AS 4970 jobs and learned the hard way. We do a 15-minute toolbox talk at site establishment for every site worker who will be on the block during the build — concretor, plumber, electrician, framer. Five minutes saves a $200,000 replacement liability later. We bring printed TPZ diagrams and stickers for the site shed.
Cardinia AS 4970 questions.
What does AS 4970 actually require?
Calculate the TPZ (12 × DBH, 2–15m range) and the SRZ. Fence the TPZ with 1.8m steel mesh, signed, before any site work. No excavation, fill, stockpile, traffic or washout inside the TPZ. Watering regime through the build. Project Arborist supervises any genuine incursion. Sign-off at four standard milestones.
What does the fence have to look like?
1.8m steel mesh panels with concrete feet, not domestic star pickets and bunting. Mulch the ground inside to 100mm before fencing. Up from site establishment, down at practical completion only.
What’s the watering schedule?
50mm per square metre of TPZ, monthly Apr–Sep, fortnightly Oct–Mar. Deep-soak via soaker hose or slow-flow sprinkler. For a 10m TPZ that’s about 15,000L per summer event — large, but small against the replacement cost of the tree.
Who signs the project off?
The Project Arborist (AQF Level 5 minimum) named in the AS 4970 report. Four inspections: pre-construction, demolition/earthworks, service trenching, practical completion. Documentation lodged with Council if the permit conditions require it. We hold that AQF Level 5 capacity in-house.
Got a planning permit with a tree retention condition?
Send us the permit reference and the plans. We’ll come back with the AS 4970 report, the TPZ fencing quote, the watering schedule and a fixed price for the four inspection milestones — all in one package.