Wilko: If any part of the deck is over 1 metre above ground level, a building consent is required. Also, a deck that is more than 2 metres wide from the house requires bracing to the sub-structure, which has to be calculated by someone who knows how to do the calculations!
Even though a permit may not be required, the structure still has to comply with the Building Act. Usually you need to use NZS 3604 as the guide for pile spacing, and bearer and joist sizes and spacing. If there is greater than 1 metre from any of the deck edges to the ground, an approved handrail must be included.
I think that both Placemakers and Mitre 10 have pamphlets which help with building a simple deck, with suggested sizes of joists and bearers. That may give you an idea of how to go about planning and building what you want.
Also, there may be resource consent issues if the additional area of the deck takes the total site coverage over that allowed by the Council's district plan. If you are on a cross-leased section, there may be requirements for obtaining the other people's approval and the plan of the deck may have to be added to the Survey Title (which means lawyers!!).
The rules were changed last year I think. You can build up to 1.5m off the ground now without consent.