With moderately durable species like the cypresses the degree of exposure is important. If the wood is wet frequently and remains damp for long periods the chances of decay within 50 years are relatively high.
Verandah posts and beams may be wet occasionally when there is rain but the surfaces exposed to wetting are usually vertical, are well ventilated and clearly visible. Therefore the risk of decay developing is relatively low. There are plenty of examples of old buildings with kauri and rimu verandah structures that are more than 50 years old.
If end grain is exposed to wetting, significant water penetration is likely. This will increase susceptibility to decay and additional protection, either with some sort of cover or with regular application of a water-repellent coating, would be advisable.
Macrocarpa (Cupressus macrocarpa), has heartwood that is in Australasian durability Class 3 i.e. in testing of ground contact 50 x 50 mm stakes it has been shown to have an average life of 5-15 years. The average life is towards the upper end of this range but a few early failures occurred in the tests. Rimu (Dacrydium cupressinum) and kauri (Agathis australis) heartwoods have the same durability classification.
Away from ground contact but fully exposed to the weather the average life of 50 mm thick cypress heartwood is 15-25 years. This is similar to the durability that could be expected from timber treated to the H3.1 specification with light organic solvent preservatives (LOSP) although there is usually more variability in naturally durable timber than in treated radiata pine sapwood.
Horizontal, upward facing surfaces, end grain and joint areas where water can be trapped are most susceptible to decay. If the surfaces are only partly exposed to the weather, are vertical or steeply sloping, and not end grain, the chances of decay are relatively low and a much longer service life is likely. 100 x 100 mm verandah support posts on steel brackets and with the upper ends protected by an overhanging roof are likely to have a service life several times greater than that of fully exposed 50 mm thick material.