We have derived star formation rates (SFRs), gas-phase oxygen abundances and effective dust absorption optical depths for a sample of galaxies drawn from the Stromlo-APM redshift survey using the new model of Charlot & Longhetti (CLO1), which provides a physically consistent description of the effects of stars, gas and dust on the integrated spectra of galaxies. Our sample consists of 705 galaxies with measurements of the fluxes and equivalent widths of Hα, [OII] and one or both of [NII] and [SII]. For a subset of the galaxies, 60- and 100-μm IRAS fluxes are available. We compare the star formation rates derived using the model with those derived using standard estimators based on the Hα, the [OII] and the far-infrared luminosities of the galaxies. The CL01 SFR estimates agree well with those derived from the IRAS fluxes, but are typically a factor of ~3 higher than those derived from the Hα or the [OII] fluxes, even after the usual mean attenuation correction of A_Hα=1mag is applied to the data. We show that the reason for this discrepancy is that the standard Hα estimator neglects the absorption of ionizing photons by dust in HII regions and the contamination of Hα emission by stellar absorption. We also use our sample to study variations in star formation and metallicity as a function of galaxy absolute b_J magnitude. For this sample, the star formation rate per unit b_J luminosity is independent of magnitude. The gas-phase oxygen abundance does increase with b_J luminosity, although the scatter in metallicity at fixed magnitude is large.