An investigation can take up to 12 months to complete, a spokesperson said.
The injured man is a “long-time” employee at Directionz, a signs and road asset installation company.
General manager Stephen Lincoln told the Rotorua Daily Post earlier this week that the incident involved a transformer and the person had hit a “high-voltage cable” while digging the foundation for a sign.
Unison relationship manager Danny Gough said Unison staff weren’t involved in the work being carried out when the incident happened but were called to the scene after the incident to secure the area and make it safe.
A Hato Hone St John spokesman said two ambulances, one helicopter, and one rapid response vehicle were sent to the scene.