You will not need to pre-arrange long-term accommodation before landing in Auckland. (Doing that would be difficult because you obviously can't visit the property, and you probably wouldn't get what you want.) However, it would be wise to book at least one night in a hotel or similar, so you have somewhere to stay while you sort out your own accommodation.
The best place to find properties to rent in New Zealand is on Trade Me, in the Property / To Rent section. You can select criteria such as location, type, price, and number of bedrooms to narrow down your search. Properties listed on Trade Me are offered by either an individual owner, or by an agency which manages the property. In either case, you can contact the relevant person on Trade Me after logging in to Trade Me.
Trade Me is also generally useful for finding all kinds of new and used goods that you might need upon arrival, often at good prices.