What is the best NZ grown timber to use for decks and does it need to be treated?

Any softwood needs to be treated for use as decking except macrocarpa heartwood. Its natural durability can be sufficient for this purpose (although its durability is unreliable in moderate-high decay hazard situations, and occasional failures may occur within 10-15 years).

Most hardwood is naturally durable and resists all kinds of wear and tear. Locally grown eucalypts can be suitable for decking.

The Decking How-to-build guide suggests treated Radiata Pine.