New Zealand Building Code Acceptable Solution E2/AS1 paragraph 9.1.8.4(d) says that timber cavity battens shall comply with the durability requirements of Acceptable Solution B2/AS1.
NZS 3602:2003 Timber and wood-based products for use in building Part 1, as modified by B2/AS1 paragraph 3.2.2, is an Acceptable Solution for meeting the durability requirements of timber and wood-based building elements.
H3.1 cavity battens
Radiata pine battens treated to H3.1 in accordance with NZS 3640:2003 Chemical preservation of round and sawn timber are deemed to meet a 50-year minimum durability performance (B2/AS1 Table 1A).
Treatment to a higher level, i.e. H3.2, is therefore not required. However, the basis of E2/AS1 is that the cavity should remain dry.
A few things to remember
Cavity battens must not be ripped down from larger timber members, as this is likely to expose untreated timber. NZS 3602:2003 paragraph 109.3 says that treatment of timber battens must be carried out in their final shape.
If the treatment is copper-based (for example CCA, copper azole or ACQ), the battens must not be in direct contact with metal wall cladding. This may cause corrosion of the cladding. A suitable separation layer must be used, such as an additional layer of paper-based underlay over the cavity battens or strips of paper-based underlay on the face of the cavity battens.
Although E2/AS1 doesn’t require a separation layer when timber has been treated with the less corrosive CuN treatment, it is prudent to still use a separation layer.