Welcome to the Bunnings Workshop community @ordgardener. It's fantastic that you've joined us and many thanks for your question.
Since Steri-prune is Petroleum Asphalts, I would advise you to use a throwaway brush for application. You can try cleaning a natural bristle brush immediately with a solvent-based thinner. A solvent-based thinner will probably adversely affect the bristles on a synthetic brush. However, once the asphalt has dried, it is unlikely to be removed at all. Even with successful cleaning, the brush will not be suitable for painting and likely relegated to only applying Steri-prune from then on.
Please let me know if you need further assistance or have questions.