If someone died, was seriously hurt, or something dangerous happened that could have killed someone, it's probably notifiable. Call the regulator straight away. Don't wait for the report to be finished. They want to know now, not next week.
Every Australian state and territory and New Zealand has its own definition. They're close, but not the same. Victoria always wants written notice within 48 hours. The model WHS states want it if they ask. New Zealand talks about notifiable events. The free tool shows the definition for your state and links straight to the regulator.
Once it's notifiable, the site stays as it is until an inspector releases it. You can help the injured, make it safe and do what police ask. That's it. Write down who released the site and when. Inspectors notice when that line is missing.
What happened, when, where, who was there. In order. Photos, sketches with measurements, statements taken while memories are fresh, and the records that matter, such as training, maintenance, permits and the SWMS. Number every piece of evidence so the report can point to it.
A sketch with a tape measure beats three paragraphs of description. Every time.
"Human error" and "needs more training" are where lazy investigations stop. A regulator wants to see what made the error possible. Run a 5 Why, test each answer, and stop at the last cause you can actually control.
Work down the hierarchy of control. Eliminate, substitute, isolate, engineer, then procedures and PPE. Give every action an owner and a date. Record who you consulted, such as HSRs and the workers who do the job. Then get it reviewed and signed off.
This guide is general information, not legal advice. Check your regulator's current requirements.