An advantage of many Australian plants is that they have small leaves and flowers or large flowers consisting of a great many small parts, where pest and disease damage tends to be minimal in terms of visual impact.
Generally, people who grow Australian plants are interested in the environmental aspect and consequently do not want to use chemicals to control pests and diseases. Their reward is the fact that those same plants tend to attract birds and beneficial insects that keep the pests in check. To maintain this natural balance, you should have enough different plants to ensure there are flowers in all four seasons of the year.
For further information on managing pests and diseases, you may like to visit the Department of Agriculture and Food website.