What is a constitution? A constitution is the fundamental legal foundation upon which a nation's government is built upon. It is a comprehensive guideline and blueprint for how the country and its legal system is to be organized and operated. It differs from a law, in that a law can be changed relatively easily by Parliament. A constitution on the other hand, can usually only be changed through a more difficult, slower process (such as a Parliament super majority or a nation-wide referendum).
A constitution almost always includes a bill of rights, which outlines the key rights all citizens possess, which a government must not interfere with. It is a way of restraining the ability of the government to oppress and excessively regulate its own people. These rights are not subject to changing laws and consensus, and so thus are a safeguard against the abuse of government power. It brings stability and order to the nation and provides protection of the freedoms and liberties of all people, to prevent the tyranny of the majority over the rights of the minority and individual.
New Zealand is only one of four countries that doesn't have a written constitution (the other three being the United Kingdom, Israel, and Saudi Arabia). Our unwritten "constitution" is taken from a variety of different sources, including Acts of Parliament, Judicial rulings, convention, and arguably the Treaty of Waitangi. However, this incomprehensive "constitution" is far too flexible and ill-defined to be useful. It doesn't adequately fulfill the two primary functions of a real constitution of providing a clear outline of the powers and responsibilities of the government and protecting the rights of citizens. Some of our laws are entrenched (meaning, like with a constitution, something more than a simple Parliamentary majority is needed to change it), such as certain sections of the Electoral Act of 1993. But even with the Electoral Act, the entrenched sections are not entrenched themselves. In addition, our Bill of Rights Act of 1990, arguably one of our most important pieces of legislation, could be abolished through a simple Parliamentary majority!
New Zealand should become a republic in order to become independent and to draw up a constitution to prevent government abuses and safeguard our rights and freedoms.