  The recently discovered massive binary system Apep is the most powerful
synchrotron emitter among the known Galactic colliding-wind binaries. This
makes this particular system of great interest to investigate stellar winds and
the non-thermal processes associated with their shocks. This source was
detected at various radio bands, and in addition the wind-collision region was
resolved by means of very-long baseline interferometric observations. We use a
non-thermal emission model for colliding-wind binaries to derive physical
properties of this system. The observed morphology in the resolved maps allows
us to estimate the system projection angle on the sky to be $\psi \approx
85^\circ$. The observed radio flux densities also allow us to characterise both
the intrinsic synchrotron spectrum of the source and its modifications due to
free--free absorption in the stellar winds at low frequencies; from this we
derive mass-loss rates of the stars of $\dot{M}_\mathrm{WN} \approx
4\times10^{-5}$ $\mathrm{M}_\odot$ yr$^{-1}$ and $\dot{M}_\mathrm{WC} \approx
2.9\times10^{-5}$ $\mathrm{M}_\odot$ yr$^{-1}$. Finally, the broadband spectral
energy distribution is calculated for different combinations of the remaining
free parameters, namely the intensity of the magnetic field and the injected
power in non-thermal particles. We show that the degeneracy of these two
parameters can be solved with observations in the high-energy domain, most
likely in the hard X-rays but also possibly in $\gamma$-rays under favourable
conditions.
