Tree risk assessment in Australia sits in an unusual regulatory position. The duty to manage the risk is real and enforceable. The qualification to assess it is not regulated at all. Understanding that gap is the beginning of understanding what you are actually buying.
“Arborist” is not a licensed title
There is no Australian licensing regime for the word “arborist”. Anyone may use it. There are genuine qualifications — AQF Level 3 and Level 5 arboriculture, and risk-specific certifications such as QTRA and TRAQ — but none is legally required to write a tree report, and local government requirements vary. Sunshine Coast Council, for instance, does not mandate AQF Level 5 for report authorship.
The practical consequence: the credential on the report matters more than the job title, and you have to ask.
What QTRA does
Quantified Tree Risk Assessment applies conventional risk-management principles to trees. Rather than producing an opinion about a tree's condition, it produces an annual probability that the tree will cause harm — a figure directly comparable to the everyday risks that regulators and courts already work with.
Three inputs are combined:
- Target. What is beneath or within range of the tree, and how often it is occupied. A footpath used twice a week and a children's playground used daily are different targets even under an identical tree.
- Size. The diameter of the part that could fail, across four ranges from 25 mm to over 450 mm. A falling twig and a falling scaffold limb are not the same event.
- Probability of failure. The likelihood that the part fails within twelve months, across seven ranges, based on visual assessment of structure, defects, decay and loading.
The three ranges are combined by the QTRA calculator, which uses Monte Carlo simulation to derive a mean, and the result is reported to one significant figure.
The thresholds
The output is read against published tolerability thresholds:
| Risk of Harm | Region | What it means |
|---|---|---|
| Greater than 1/1,000 | Unacceptable | Action required |
| 1/1,000,000 to 1/1,000 | Tolerable | Manage and monitor; risk imposed on others warrants closer consideration above 1/10,000 |
| Less than 1/1,000,000 | Broadly acceptable | No action indicated; re-inspect on cycle |
The 1/10,000 line inside the tolerable region is the one that does most of the work in practice. It marks where risk is being imposed on people who did not choose to accept it — guests, residents, passers-by, workers — rather than accepted voluntarily.
Why a number rather than a rating
Because the duty you are discharging is expressed in the same terms. Section 19 of the Work Health and Safety Act 2011 (Qld) requires a person conducting a business or undertaking to ensure health and safety “so far as is reasonably practicable”, including “the provision and maintenance of a work environment without risks to health and safety”. Reasonable practicability is a weighing exercise: the likelihood of the hazard, the degree of harm, what is known about it, the availability of controls, and their cost.
You cannot weigh a word. You can weigh a probability against the cost of the control that would reduce it. That is the entire argument for quantification, and it is why QTRA reports read the way they do.
QTRA is a method, not a business model. It can be applied by an assessor who also sells removals. The method is sound either way — but the recommendation flowing out of it is only as independent as the person making it. Tree Risk QTRA carries out no tree works of any kind, which means there is no version of the finding that generates additional revenue.
Who actually needs one
- Anyone with a duty under the WHS Act whose workers or visitors are beneath trees — schools, clubs, holiday parks, aged care, industrial sites, councils.
- Bodies corporate, whose obligations attach to common property.
- Property purchasers, because standard pre-purchase building inspections explicitly exclude landscaping.
- Anyone in a dispute — with a neighbour, an insurer, or a council — where the condition of a tree is contested.
- Anyone applying to remove a protected tree, where a quantified assessment demonstrates that removal was not the first option considered.