  Emission line spectra from HII regions are often used to study properties of
the gas in star-forming regions, as well as temperatures and luminosities of
the ionising sources. Empirical diagnostics for the interpretation of
observational data must often be calibrated with the aid of photoionisation
models. Most studies so far have been carried out by assuming spherical or
plane-parallel geometries, with major limitations on allowed gas and dust
density distributions and with the spatial distribution of multiple,
non-centrally-located ionising sources not being accounted for. We present the
first results of our theoretical study of geometric effects, via the
construction of a number of 3D photoionisation models using the MOCASSIN code
for a variety of spatial configurations and ionisation sources. We compare
integrated emission line spectra from such configurations and show evidence of
systematic errors caused by the simplifying assumption of a single, central
location for all ionising sources.
