I'm not from NZ (and I don't live there yet..), so could you tell me if you think this verdict could have implications for places like the Tongariro Alpine Crossing (active volcanic hikes or things like that).
Hard to say in specific terms. Generally, we've already seen changes to the Adventure Activities Act (after much debate and industry consultation) which will put a lot more emphasis on informed consent and risk disclosures for clients. There's also more emphasis on risks due to major natural events and having systems to identify and mitigate these.
Honestly, the chances to the Act have been pretty minimal, and I don't think this verdict changes that. What it does achieve is force the hand of shonky operators to come up to standard.
Most of the people who do the Tongariro crossing are self guided, so nobody is responsible for them but themselves. Organisations like outdoor clubs are explicitly written out of the legislation for recreation.
This is explicitly a workplace safety ruling, so only applying to guided trips.