Delayed PTSD and dual employer liability: Insights from BHP v Thiess

In the recent decision of BHP Nickel West Pty Ltd v Thiess Pty Ltd [2026] WADC 11, the District Court of Western Australia upheld an Arbitrator’s finding that a previous employer may still be liable for a disease that was not diagnosable during the previous employment, and may not have manifested, but for a later event with the subsequent employer.

Background

In 2021, the worker was employed by BHP as a dump truck driver when she was involved in a truck accident (first accident). She sustained minor physical injuries, made a workers’ compensation claim, recovered and later left that employment.

On 26 November 2022, while working for Thiess as a dump truck driver, she was involved in another accident (second accident). Although she suffered no physical injuries, she developed psychological symptoms that were later diagnosed as Post Traumatic Stress Disorder (PTSD).

The worker commenced a Workers Compensation Arbitration against Theiss. By the time of arbitration, she had still not returned to work.

Thiess initially denied liability, but argued in the alternative that, if liable, responsibility should be apportioned with BHP on the basis that the PTSD was caused in part by the first accident. Thiess joined BHP as a ‘relevant employer’ pursuant to section 34(5)(a) of the Workers’ Compensation and Injury Management Act 2023 (WA) (Act).

Arbitrator’s decision

The arbitrator found that, although the worker did not meet the DSM‑5 criteria for PTSD following the first accident, the earlier event was still capable of being a causative traumatic incident. He accepted that PTSD could have delayed expression and noted that the worker displayed early symptoms, particularly hypervigilance, after the first accident, which intensified after the second.

Evidence of three psychiatrists agreed that the diagnosed PTSD related to both accidents, though they differed in how they assessed each accident’s causal contribution.

The arbitrator preferred the evidence of Drs Shub and Lee (psychiatrists), who concluded that both accidents caused the PTSD, with the second operating as the ‘tipping point’ that unmasked significant psychological injury originating from the first accident.

Although not explicitly discussed in the decision, the Arbitrator appeared to affirm Pollock v Wellington [1] in finding that the third psychiatrist’s opinion carried less weight because his opinion relied on an incorrect assumption in forming his view.

Ultimately, the arbitrator found the worker did not have PTSD before the second accident and probably would not have developed PTSD had the second accident not occurred.

Accordingly, the arbitrator held Thiess liable to the worker but ordered BHP reimburse 50% of the compensation, pursuant to section 34 of the Act.

Appeal

BHP argued that the PTSD was not contracted during its employment because the diagnostic criteria were not satisfied until after the second accident. BHP therefore submitted it could not be a ‘relevant employer’ under section 34.

Thiess submitted that the worker exhibited symptoms of PTSD after the first accident and prior to the second accident. That PTSD was not diagnosable until after the second accident did not prevent the first accident from being treated as a causative event for the purposes of liability under section 34.

Court’s findings

The Court identified the key issue as whether a previous employer may be liable where a disease is not diagnosable during that employment and may not have developed but for a later event.

A disease is an injury from employment if it is contracted in the course of that employment and the employment contributed to a significant degree [2]. The Court held that, in the context of a non-infectious disease such as PTSD, ‘contracted’ carries its ordinary meaning, that is, to develop or acquire, and does not require that all diagnostic criteria be met at that time.

Turning to section 34, the Court confirmed that apportionment between employers was only available where the injury ‘is or may be from’ another period of employment. This requires evidence that the disease was contracted during that earlier employment.

On the facts, the Court accepted that the PTSD was diagnosed only after the second accident, but the expert evidence established that PTSD could have delayed expression, with diagnostic criteria sometimes not met for many months after a traumatic event. This made it open to the arbitrator to find that the worker contracted delayed onset PTSD as a result of the first accident, which was then triggered by the second accident. Both events therefore contributed to the disease.

Accordingly, the Court held that the arbitrator did not err in apportioning liability between the two employers.

Implications

While each case will turn on its own facts, this decision makes it clear that a worker may have a compensable injury if they contract PTSD contributed to during earlier employment even if the condition is not diagnosable (becomes manifest) until after a later event.

That later event may be in the course of, or arise out of, employment with a later employer but liability can still be apportioned (to the earlier employer) if the earlier event was a cause.

Employers should therefore monitor and manage early psychological symptoms and be alert to the possibility that delayed‑onset conditions like PTSD can create multi‑employer exposure and apportionment.

Please note that this case is pending appeal.