Why pre-emerge instead of early pre-plant (or early post-emerge)?  Soil residual chemistries won't do any good lying on top of the soil.  For corn, in a climate such as yours, I'd think it best to get the residual products out there soon enough to up the odds of getting them rained into the soil.*  Either that, or hold onto them for post-emerge and try not to apply just after a rain (lest some weeds be germinated but not yet emerged enough to catch spray droplets). As far as triazines killing grasses, you're more likely to accomplish the job w/ foliar application (with oil) when the grasses are small, as opposed to purely as soil residuals.

*I'm guessing that your climate history has decreased odds of rain during corn planting as opposed to earlier, such as during chickpea planting.  Note the word 'guessing.'  But most of Oz cropland is in a Mediterranean climate, is it not?