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".

Agree with previous response. I'd just add a comment that you should ask yourself who is the signage for? If the space is a permanent one and there is risk of someone inadvertently (or even deliberately) entering (and it is reasonably practicable to do so, which more often than not I'd argue it is) then it should be signposted. The PCBU in management and control of that space has then taken a reasonable step (one of potentially many others) in advising workers and other persons of the hazard. If the confined space only exists for a (short) period of time, say for example a temporary access chamber into a sewer, and it is constantly manned during the work (safety watch on standby for the full duration of works) then I'd suggest a sign is of little to no value. In my experience across NZ there are thousands of permanent confined spaces that have no signage installed by the asset owner and it then falls onto the PCBU engaged to undertake work in that space to provide their own (temporary) signage where relevant and practicable whilst work is being carried out.