This, like many questions like it, depend on what you define as legally obliged. While HSWA emphasizes the importance of identifying hazards and managing risks (like those associated with confined spaces), it does not explicitly mandate the posting of signs at the entry points of confined spaces.

However, WorkSafe New Zealand accepts the Australian Standard AS 2865 for confined spaces as the current state of knowledge and practice. This standard recommends that confined spaces should be clearly identified and signposted to warn workers of the potential hazards.

So, while it may not be a strict legal obligation under a specific New Zealand Act or Regulation, the question to really ask is, "is following the best practices outlined in AS 2865, including signposting, reasonably practicable?" If so, then you are "legally obliged".