The outcome of WorkSafe's Whakaari / White Island investigation is expected in the next week.
More than 25 fulltime WorkSafe staff have been involved in the probe into possible breaches of health and safety law, leading up to the eruption last year.
Twenty-two people died from their injuries from the explosion on December 9 and at least a dozen others suffered critical injuries.
Before the eruption, there had been eight deaths from adventure activities in New Zealand since 2014.
A WorkSafe summary document, now released under the Official Information Act, identifies duty holders in New Zealand workplaces as part of background research into the Whakaari/White Island eruption done this year.
Any duty holder can be prosecuted under the Health and Safety at Work Act following an investigation but prosecutions have to be made within a year of a workplace accident.
The document shows the "primary duty of care" lies with "a person conducting a business or undertaking" and at Whakaari that included tour operators, landowners and even emergency services.
Two tour companies - White Island Tours and Volcanic Air - had tourists and staff on the submarine volcano when it erupted.
The Buttle family was the landowner and emergency services responding after eruption included St John and NZ Police.
The document says these groups must do "what is reasonably practicable to ensure the health and safety of workers is not put at risk".
The next duty holder mentioned in the document is any "officer".
This includes directors, partners and senior leaders in businesses who have "significant influence in management" and make "policy and investment decisions that affect health and safety".
The WorkSafe summary says these officers need to exercise "due diligence" to ensure the business complies with its duties.
"Officers must take reasonable steps to understand how their business works and how it manages work health and safety."
It says workers are also duty holders.
They have to take "reasonable care" for their own health and safety, the document states, and co-operate with their workplace policies and procedures and comply with instructions, to avoid harm to themselves and others.
Finally, visitors, customers and volunteers are also duty holders, the document states.
They need to comply with instructions and take reasonable care for their own health and safety and avoid adversely affecting others.
WorkSafe investigations normally entail a scene examination and evidence gathering, interviewing people, reviewing documents related to the work operation, engaging experts to help WorkSafe understand what happened and why and engaging with other agencies.
When the investigation is completed, WorkSafe then decides whether to prosecute.
The health and safety regulator can lay charges, often leading to convictions and hundreds of thousands of dollars in fines and reparations, determined by a judge.
WorkSafe can also refer the incident to another agency, it can provide a report to the Coroner and ensure immediate action is taken to reduce harm.
In a statement this week, a WorkSafe spokeswoman said: "Multiple duty holders can be charged in relation to a single incident."
WorkSafe cited last year's prosecution against a building company and an engineering company in Invercargill as an example.
Phil Stirling Building Ltd and Duncan Engineering Ltd were both fined, convicted and ordered to pay reparations over the same incident, where two workers were seriously injured while building a milking shed in Southland in 2017.
A WorkSafe investigation found the companies failed to ensure other workers onsite knew to keep clear of the risk area.
NZ Police staff are investigating Whakaari / White Island deaths on behalf of the Coroner and this is expected to continue into 2021.