There are different rusts for different plants. It generally looks like small yellow-orange pustules.

First up, consider how bad the infection is, is it on something like a frangipani that is about to lose all its leaves anyway? I tend to ignore rust on deciduous trees - it usually comes at the end of the season and not much makes it better anyway. In general, airflow improves and reduces the occurance and general garden hygiene helps. But I also don’t waste my time bagging and binning every fallen leaf.

Myrtle rust affects plants in the Myrtaceae family. It is an exotic disease that is now affecting our native plants and quite serious. If you think you have myrtle rust, consult your local nursery for a suitable product, or consider removing the plant and disposing of appropraitely to prevent spread.