This study tested the effects of Spirulina supplementation level, basal diet and their interactions with sire breed and sex on liveweight, body conformation and growth traits in dual-purpose Australian lambs. In two concurrent feeding trials utilising typical pasture-based and simulated-drought basal diets, a total of 48 lambs was randomly allocated to treatment groups of Spirulina supplementation levels (CONTROL – 0 ml, LOW – 50 ml, MEDIUM – 100 ml and HIGH – 200 ml), balanced by sire breed (Black Suffolk, Dorset, Merino and White Suffolk) and sex (ewes and wethers). In both feeding trials, Spirulina was supplemented daily for 9-weeks, following a 3-week adjustment phase. Weekly data of liveweight and body conformation measurements; chest girth (CG), wither height, body length, and body condition score (BCS), were taken throughout the trials. These were transformed into changes between initial and final periods of the feeding trial. Average daily liveweight gain (ADG) was computed from the differences in liveweight divided by duration in days. All data were analysed using Factorial ANOVA analysis in SAS. Medium and high Spirulina supplementation levels were found to improve the liveweight and ADG of White Suffolk- and Merino-sired lambs on simulated-drought basal diets. BCS improved with Spirulina supplementation in lambs on simulated-drought basal diets. Under typical pasture-based basal diet, supplemented lambs had comparatively higher CG, BCS, liveweight and ADG than their counterparts on simulated-drought basal diets. These findings are of practical significance to the sheep industry because of the potential selection aid to Australian farmers in identifying the usefulness of Spirulina as a supplement and the optimal sire breed choice for the best performance response that is cost-effective when using dietary protein-rich supplements for their dual-purpose prime lamb operations.