You must be already aware that New Zealand has a point based visa system. Therefore, its obvious that there would be various elements that would collectively determine the overall score based on points obtained. English is the most commonly used language in New Zealand. It is a language that an applicant must have good functional command of. English as a language is one of the perimeters for determining the overall eligibility of a person that would result in the grant of visa. Now answering the basic question. A person, if he chooses to use IELTS for providing sufficient language proficiency proof, should score a minimum of 6.5 in case he is the principal applicant and in case of dependents, it should be a minimum of 5. Let us remind you that these are the minimum scores and scoring higher would have a positive impact on your overall New Zealand immigration points IELTS.
Not necessarily, there are many other standardized tests that you can take for demonstrating your proficiency in the language for the purpose of visa application. Besides IELTS, you can also opt for taking TOEFL, Pearson Test of English academic, FCE & OTE tests. Their scores are also considered for evaluating your visa application points.
Minimum scores for the mentioned options are as follows:
TOEFL: 35 or more.
PTE: 36 or more.
FCE: 154 or more.
OTE: Grade C or higher in all the four sections.
Hope the above-mentioned information served its purpose. However, it is always recommended to take advice of a licensed immigration expert.