Purpose Understorey Acacia spp. plays an important role in post-fire restoration because these understorey plants are tolerant to stress conditions. We investigated how the ecophysiological status of two species of understorey, Acacia leiocalyx and Acacia disparrima, varied depending on the plant growth stage after prescribed burning. Materials and methods Plants were grouped in different size classes, namely seedlings, small and medium sizes, and physiological variables such as foliar gas exchange, water use efficiency and light dependency were measured at two experimental sites subjected to prescribed burning. Results and discussion A. leiocalyx showed higher symbiotic N2 fixation and photosynthetic capacity compared to A. disparrima regardless of plant-size classes at both experimental sites. This could explain the greater relative growth rate of A. leiocalyx than that of A. disparrima. However, A. disparrima is more tolerant to shady conditions than A. leiocalyx. Conclusions This finding may be an indication of how well these two species recover after fire, although A. leiocalyx may have faster regrowth, as it is fixing more N.