Wet boric or boron salts timber treatments were first introduced in 1952 – before that, native timber and some of the first radiata pine framing was used untreated. From 11 September 1995, NZS 3602:1995 Timber and wood-based products for use in building allowed the use of untreated timber for framing provided it was kiln-dried and that its in-service moisture content did not exceed 18%. The change in the standard was cited in Building Code compliance document B2/AS1 on 28 February 1998. (Any use of untreated kiln-dried timber before this would have to have been consented as an Alternative Solution.)
On 9 March 2003, the Building Industry Authority (BIA – the forerunner to both MBIE and DBH) issued BIA directive 23, which required that treated timber be used for all consents issued from 1 April 2004. Consents already issued that included use of untreated timber remained valid as long as the buildings were completed before 1 April 2005.
In April 2011, Amendment 7 to B2/AS1 further amended the treatment requirements of NZS 3602 to allow the use of H1.2 boron-treated radiata pine and Douglas fir framing within a closed space, except for cantilevered balcony floor joists and associated enclosed balcony wall framing where H3.2-treated timber was required. The amendment allowed the use of untreated Douglas fir in buildings with very low weathertightness risk.
Summarised from BRANZ Guideline September 2013.