Long-term ex-situ seed storage under controlled conditions in gene banks has become an important tool for conserving threatened Australian plants; however, there is scant information about the seed longevity of most species. The aim of the present study was to determine whether the seed longevity of two contrasting Australian species could be modelled using the seed viability equation, and whether the universal temperature constants are applicable to these species. Seeds of Eucalyptus erythrocorys F.Muell. (Myrtaceae) and Xanthorrhoea preissii Endl. (Xanthorrhoeaceae) were aged at moisture contents ranging from 3.9 to 15.7% and temperatures between –20 and 60°C. Survival data were fitted to the seed viability equation in one step and the species constants for each species determined. Both E. erythrocorys and X. preissii seeds exhibited orthodox seed storage behaviour whose longevity could be modelled using the seed viability equation. The viability constants were KE = 8.81, CW = 4.97, CH = 0.0412 and CQ = 0.000379 for E. erythrocorys and KE = 8.77, CW = 5.29, CH = 0.0382 and CQ = 0.000473 for X. preissii. The universal temperature constants could not be used without a significant increase in error. The storage behaviour of these two Australian species is in keeping with that of orthodox species from around the world. Predictions are that E. erythrocorys will be long-lived under gene bank conditions, whereas X. preissii would be moderately long-lived. Current long-term gene bank storage conditions appear suitable for storage of these species; however, recommendations for short-term storage need to be re-evaluated.