Abstract This work presents the first experimental results on the electron properties of a nanosecond diffuse fast ionisation wave generated in synthetic dry air at atmospheric pressure under very strong overvoltage. Both density and mean temperature of electrons are investigated by incoherent Thomson scattering. The electron density is also derived from the Stark broadening of oxygen lines resolved by optical emission spectroscopy. The extreme voltages applied question some common hypothesis of the diagnostics implemented. The solutions adopted and the remaining limitations are discussed in the paper. Each diagnostic covers a specific region of interest within the discharge and they show good agreement in conditions where they overlap. It is shown that most of the volume of the pin-to-plane discharge is quite representative of a quasi-steady state glow discharge dominated by the emission of the first and second positive systems of nitrogen. Once its propagation completed within the first two nanoseconds and until the end of the 10 ns pulse, it is characterized by rather homogeneous properties close to the axis. The electron density is of the order of 10 15 cm −3 and the mean temperature is about 3 eV within the whole air gap. About 6 ns after the start of the discharge from the pin, a sub-millimetric region of strong ionization develops at the pin, which is consistent with the observation of a continuum of emission spreading from the UV to the near-IR spectral range. Within this part of the discharge, the electron density reaches values greater than 10 17 cm −3 with an ionization degree higher than 1%. The radiative recombination of nitrogen ions N 2 + and the three-body recombination of N + with a large number of electrons could help to explain the continuum.