Documents required for travel to New Zealand - Baby?

Grandpa and myself are visiting New Zealand from South Africa for 6 weeks in December. Her parents are joining us 2 weeks later. I need urgently to know what travel documents (besides passport) is required for me to have permission to enter and exit the country with the infant. I do not want any trouble at the customs for a 5 month old baby that is accompanying us without her parents. I have sent faxes, emails and made telephone enquiries with the New Zealand High Commissioner in Pretoria, South Africa - to no avail (since 5 November). I really hope there is someone who can help me on this urgent issue.