If you don’t hold a practising certificate – but have been admitted to the High Court - you may, under certain circumstances, certify true copies, witness signatures, or take an oath or declaration.
If you have been admitted to the New Zealand High Court as a barrister and solicitor but do not hold a current practising certificate, you may provide legal services, but not in the 'reserved areas of work' as defined in s6 of the Lawyers and Conveyancers Act (LCA).
You can also do certain things which are, by statute, able to be done by 'an enrolled barrister and solicitor'. These include taking statutory declarations.
If you provide any legal services without a current practising certificate you must not describe yourself as a lawyer - or law practitioner, legal practitioner, barrister, barrister and solicitor, solicitor, attorney-at-law or counsel. To do so is an offence under s21 of the LCA. You should describe yourself in a way that will not be in breach of the LCA. One suitable formula might be 'Enrolled barrister and solicitor of the High Court of New Zealand'.
Note, though, that the situation is different for lawyers admitted in other jurisdictions.