Around 50 people were on White Island -- also known as Whakaari -- in December 2019 when a deadly column of burning ash and steam blasted from a volcanic vent.
The fatal eruption also left 25 people with horrific burns.
A court in Auckland found Whakaari Management failed to "assess risk" to visitors sufficiently, or provide them with protective equipment.
"This was critical to ensuring that tours could be conducted safely" said Judge Evangelos Thomas.
A second charge of ensuring the safety of those working on the island was dismissed.
The management company was the last of 13 individuals and organisations originally charged over the disaster.
Six of the 13, including the three brothers who own the island, had already been acquitted.
Whakaari Management -- which granted permits to visit the island -- will be sentenced next February with six other parties who pleaded guilty.
The guilty parties face fines of up to NZ$1.5 million (US$876,652).
During a 10-week trial, the court heard that the island's owners made around NZ$1 million (US$620,000) a year from tourists before disaster struck.
In May last year, New Zealand's emergency management agency was cleared of any wrongdoing.
Since the eruption, no boat or aircraft tours have been allowed to land on the island.
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