Motivated by predicting the performance and reliability of piezoelectric devices, considerable research efforts have been made in recent years to mathematically describe the largesignal nonlinear behaviour of ferroelectric piezoceramics. The success of such modelling approach depends strongly on our knowledge of the loading history dependence of the material properties. In this experimental work, the nonlinear ferroelastic behaviour was investigated in detail for a commercially available soft lead zirconate titanate (PZT) material under low-frequency compressive stress loading. It was found that the strain response was significantly loading rate dependent. The evolution of the elastic moduli was determined using fast small-stress partial unloading method, which was specially designed to minimize the time-dependent effect as much as possible. The results were used to separate the reversible elastic deformation from the irreversible plastic strain due to domain switching.