The research deals with nonlinear properties of piezoelectric, ferroelectric and mechanical response of the lead-free type ferroelectric ceramics x(Bi0.5 Na0.5) TiO3-y(Bi0.5 K0.5) TiO3-zBaTiO3, where x 0.79, y 0.14, and z 0.07, in short form BNBK79. Small mechanical deformation and polarization measurements are realized using LVDT (Linear Variable Differential Transformer) measuring system. Velocities of ultrasound wave propagation in presence of an external DC electric field are also determined. The orientation of spontaneous polarization can be switched under the action of sufficiently strong electric fields. This causes the nonlinear behaviour of polarization-electric field (D-E), strain-electric field (S-E) and the velocity of ultrasonic waves-electric field (v-E) relationship. Butterfly shape of the responses is observed above the coercive field. The described BNBK79 ceramics is significant candidate for lead-free actuator applications.