Minimise exposure. Staying indoors is probably the most effective. Bushfire particles are extremely small, 2.5 microns. Paper masks are pretty useless. They could give a false sense of protection so still keep exposure/outdoor activity to a minimum. Respiratory protection masks rated at P2 or higher offer some protection. The masks you buy at the hardware shop for protection against paint fumes. Note the seal needs to be air tight. If it leaks, it’s no protection. Beards make it near impossible. Air pollution isn’t just a nuisance, it can increase mortality and impact health.
https://www.abc.net.au/news/health/2019-12-03/face-masks-for-bushfire-smoke-haze-pollution/11760618
https://www.health.nsw.gov.au/environment/factsheets/Pages/bushfire-smoke.aspx