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An above-ground flat-panel test was conducted at Innisfail to determine the influence of tree age on natural durability against decay of heartwood of eucalypt species grown in low-rainfall areas. After nine years’ exposure, heartwood from trees aged 30-50 or 80+ years had similar natural durabilities in Eucalyptus sideroxylon, Corymbia spp., E. cladocalyx and E. occidentalis. However, except for E. cladocalyx, the heartwood from trees less than 25 years old was less durable than that from older trees for Corymbia spp., E. leucoxylon, E. occidentalis and E. sideroxylon. Density was a useful predictor of natural durability, especially in the lower range that differentiated wood in young trees. The importance of density within the more-mature tree age ranges of 30-50 years and 80+ years was unclear, perhaps showing that a threshold age for the production of mature heartwood had been reached. Comparisons, including with the yardstick species Pinus radiata, E. regnans, E. obliqua and E. camaldulensis, suggest that E. occidentalis could be assigned a class-2 above-ground natural durability rating in AS 5604, while the class-1 rating for E. astringens should be re-examined.