A self-drive vehicle to enhance worker safety will be trialled by one of the country’s largest electricity providers, as part of a new agreement with WorkSafe New Zealand.

Mercury NZ has signed the commitment in response to the uncontrolled release of geothermal steam at its Rotokawa power station near Taupō in July 2021.

While the so-called ‘steam hammer’ event did not injure anyone, it could have seriously harmed workers if they had been in the area at the time. Steam hammer is created when steam meets cooler liquid in pipework and fittings, causing severe vibration that can lead to catastrophic failure.

A WorkSafe investigation found shortcomings in plant installation decisions and risk assessment by Mercury, and ineffective communication between offsite control room operators and personnel onsite.

“The loss of containment was incredibly dangerous. The pipework had been exposed to extreme forces, with an intensity that ejected flange bolts, split valve bonnets, and tore welded fixings,” says WorkSafe’s regulatory support manager, Catalijne Pille.

“Businesses must do everything they can to meet their responsibilities under the Health and Safety at Work Act.”

In response to the incident, Mercury has now applied to WorkSafe with a binding commitment to improve safety. The plan, known as an enforceable undertaking, includes:

  • Trialling a self-driving vehicle for plant inspections
  • Delivery of a leadership programme to promote a proactive safety culture
  • Introduction of training focused on hazard awareness and safety in high-risk environments
  • Sharing the resources developed and lessons learned from the incident with industry.

“Emerging technologies have huge potential for health and safety. Mercury plans to trial self-drive vehicles to supplement in-person operator rounds which can only be good for safety. The data insights will aid decision-making and help with continuous improvement of processes and procedures,” says Ms Pille.

As a result of the agreement WorkSafe’s charges against Mercury have been discontinued. WorkSafe will regularly monitor progress on the commitments which have been agreed and can resume prosecution if necessary.

“The investment from Mercury is the preferred solution in this case. It demonstrates a substantial commitment to health and safety with benefits to workers, community, and the industry that may not have been achieved by prosecution.”

Read the decision document

Read more about geothermal safety

Statement from Mercury’s Executive General Manager Generation, Stew Hamilton

The health, safety and wellbeing of our people is front of mind for us, and we’re constantly working to improve on our performance in this space.

While no one was harmed in this event, it is important that we reflect on this incident and how we can continue to improve on our safety performance. This Enforceable Undertaking is an opportunity for us to do so, and fits into our wider vision for world class safety performance and achieving our goal of safety citizenship.

The programme has now kicked off, and we expect it to complete in February 2026. Key work within this will include further education and coaching, autonomous inspections of certain sites, mechanisms to share learnings with others, donations to emergency service providers and support of a health and safety scholarship. We expect this will deliver benefits to not just our people, but our sector as well as the community.

We are looking forward to working with WorkSafe to help continue to build strong safety performance across the industry.

Media contacts

For WorkSafe: media@worksafe.govt.nz
For Mercury: mercurycommunications@mercury.co.nz