Ministers, in the New Zealand Government, are members of parliament who hold ministerial warrants from the Crown to perform certain functions of government. This includes formulating and implementing policies and advising the Governor-General. Ministers collectively make up the executive branch of the New Zealand state.
The formal powers of the executive are exercised through the Executive Council, which consists of all ministers plus the Governor-General. When the Executive Council resolves to issue an order, and the order is signed by the Governor-General, it becomes legally binding.
A minister is generally charged with supervising a particular aspect of the government's activities, such as the provision of health services (Minister of Health) or the upkeep of law enforcement (Minister of Police). Sometimes, people may be appointed ministers without being given any specific role — they are known as ministers without portfolio. Such appointments have become rare today, although sometimes a person may be appointed to a sinecure portfolio such as "Minister of State" for similar purposes.
The appointment of ministers is officially made by the Governor-General, who must sign a ministerial warrant before it officially comes into effect. Governors-General appoint or discharge ministers on the basis of advice from Prime Ministers, who are themselves appointed by the Governor-General on the basis of whether they have the confidence of Parliament. The recommendations that a Prime Minister chooses to give are theoretically their own affair, but the political party (or parties) behind them will almost certainly have views on the matter, and most recommendations are made only after negotiation and bargaining. Different parties have different mechanisms for this - the Labour Party, for example, has provision for caucus to select ministers, while in the National Party, a Prime Minister theoretically has greater authority to make their own selections.