  The interpretation of supernova (SN) spectra is essential for deriving SN
ejecta properties such as density and composition, which in turn can tell us
about their progenitors and the explosion mechanism. A very large number of
atomic processes are important for spectrum formation. Several tools for
calculating SN spectra exist, but they mainly focus on the very early or late
epochs. The intermediate phase, which requires a NLTE treatment of radiation
transport has rarely been studied. In this paper we present a new SN radiation
transport code, NERO, which can look at those epochs. All the atomic processes
are treated in full NLTE, under a steady-state assumption. This is a valid
approach between roughly 50 and 500 days after the explosion depending on SN
type. This covers the post-maximum photospheric and the early and the
intermediate nebular phase. As a test, we compare NERO to the radiation
transport code of Jerkstrand et al. (2011) and to the nebular code of Mazzali
et al. (2001). All three codes have been developed independently and a
comparison provides a valuable opportunity to investigate their reliability.
Currently, NERO is one-dimensional and can be used for predicting spectra of
synthetic explosion models or for deriving SN properties by spectral modelling.
To demonstrate this, we study the spectra of the 'normal' SN Ia 2005cf between
50 and 350 days after the explosion and identify most of the common SN Ia line
features at post maximum epochs.
