Measurement of key plasma jet properties, such as the Mach number, electron density and temperature, in a low pressure environment, were performed using double Langmuir probes and Mach probes. In particular, under-expanded jets are studied in detail by performing complete mappings of plasma jet properties at chamber pressures of 10 and 2 mbar. These results show that the measured physical properties are consistent with the jet flow phenomenology, such as the presence of periodic expansion and compression zones, the effect of the pressure and the location of the shocks. It is shown, in particular, that for a highly under-expanded jet at 2 mbar, the Mach number reaches 2.8 in the first expansion zone followed by a strong drop to subsonic flow revealing the presence of a Mach reflection. The flow is accelerated further and a periodic structure of compression/expansion cells is observed until the local static pressure is in equilibrium with the surrounding pressure. These results contribute to the understanding of the supersonic plasma jet behaviour at low pressure and can be used to quantify the deviation from local thermodynamic equilibrium. The extensive mapping of the measured physical properties of the jet will also serve as input for modelling.