Effects of decay hazard, decay type and preservative depletion hazard on the performance of variously preservative treated 20 x 20 x 500 mm Radiata pine and Fagus sylvatica test stakes across 13 field test sites in New Zealand and Queensland Australia were determined. Radiata pine treated with an ammoniacal copper quaternary preservative (ACQ) (1.56% m/m a.i.) and copper chrome arsenate (CCA) (0.72% m/m a.i.) was susceptible to sudden failure at some of the sites that had a high brown rot hazard whereas pine treated with copper-azole (CuAz) (0.59% m/m a.i.) was not affected, suggesting that CuAz was particularly effective against brown rots. At the most severe brown rot sites ACQ treated pine was more susceptible than CCA and its performance in service may be compromised as a result, as previously occurred for pine treated with acid copper chromate (ACC). Based on overall performance across all sites pine treated with chlorothalonil plus chlorpyriphos (1.07% m/m a.i.) (11% mean soundness reduction (MSR)), creosote/oil treated pine (37% m/m a.i.) (14% MSR) and CuAz (15%MSR) all gave significantly better protection (5% P) than pine treated with CCA (19% MSR) and ACQ (19% MSR). The decay hazards encountered, as determined by mean soundness reduction after 6.5 years, were more severe than encountered in previous studies at some of the same sites and this was linked to differences of intra-site decay type between test plots and associated decay hazard differences. Greater decay rates encountered in this study were, in part attributed to high preservative depletion. At very wet sites, particularly those likely to have a high soil organic acid content, 20 x 20 x 500 mm stakes are probably too small for accurate interpretation of long-term durability of preservative treated wood. Knowledge of the distribution of different decay types across sites tested, coupled with associated decay hazards and preservative depletion hazard, suggested that 4 test sites of clearly defined features would enable comprehensive field testing of preservative treated wood. Selection of sites is not straightforward and requires a rudimentary knowledge of soil type, geology, vegetation and climate, or comprehensive knowledge of the decay types present. In view of the cost of maintaining test facilities, adequate multi-site testing of new wood preservatives may be achievable through cooperation between research establishments. Possibly, current test site selection criteria fall short of ensuring adequate test site parameters are incorporated and maximum cost effectiveness of testing may not always be achieved through duplication of test site parameters between sites of similar but unrecognised properties. Scope for artificial creation of intra-site decay hazard differences is discussed.

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