Welcome to the Bunnings Workshop community. It's wonderful to have you join us, and thank you for sharing your question about cleaning your front security screen.
I suggest trying the soft approach first before you go full blast with the pressure washer. I recommend using the brush attachment of your vacuum cleaner. I suggest doing a double pass both front and back to remove the majority of dust and debris from the security screen.
Next, I suggest using Morgan Round Dish Brush it's stiff bristles should dislodge all the stubborn dirt that did not get taken off by the vacuum cleaner. Make sure to go in an up and down direction to prevent warping the screen.
Vacuum the area again to pick up any stragglers that the brush managed to take off. My last suggestion is to use a damp cloth to wipe the screen clean. Do several passes and make sure to rinse the cloth often, I don't recommend using any chemical or soap cleaners to avoid damaging the screen.
Please keep us updated with your progress, we look forward to seeing your security screen nice and clean.
If you have any other questions, please let us know.