Abstract We present an analysis of the diffuse ionised gas (DIG) in a high-resolution simulation of an isolated Milky Way-like galaxy, incorporating on-the-fly radiative transfer and non-equilibrium thermochemistry. We utilise the Monte-Carlo radiative transfer code colt to self-consistently obtain ionisation states and line emission in post-processing. We find a clear bimodal distribution in the electron densities of ionised gas (ne), allowing us to define a threshold of ne = 10 cm−3 to differentiate DIG from ${\rm H\, \small {II}}$ regions. The DIG is primarily ionised by stars aged 5 – 25 Myr, which become exposed directly to low-density gas after ${\rm H\, \small {II}}$ regions have been cleared. Leakage from recently formed stars (<5 Myr) is only moderately important for DIG ionisation. We forward model local observations and validate our simulated DIG against observed line ratios in [${\rm S\, \small {II}}$]/Hα, [${\rm N\, \small {II}}$]/Hα, [${\rm O\, \small {I}}$]/Hα, and [${\rm O\, \small {III}}$]/Hβ against ΣHα. The mock observations not only reproduce observed correlations, but also demonstrate that such trends are related to an increasing temperature and hardening ionising radiation field with decreasing ne. The hardening of radiation within the DIG is caused by the gradual transition of the dominant ionising source with decreasing ne from 0 Myr to 25 Myr stars, which have progressively harder intrinsic ionising spectra primarily due to the extended Wolf-Rayet phase caused by binary interactions. Consequently, the DIG line ratio trends can be attributed to ongoing star formation, rather than secondary ionisation sources, and therefore present a potent test for stellar feedback and stellar population models.