Most of the renovations done in New Zealand need a building consent, which has to be applied from a building consent authority (BCA). In most of the cases the BCA will be your local council who you will demonstrate your renovation plans to. 

In some cases the building consent is not needed, but you still have to comply with the Building Code. The building consent is required for example for alterations, additions and structural repairs, construction of decks, plumbing or drainage work. Take a look at the article about building consent rules in New Zealand.