Although many Australian and particularly Western Australian plants have evolved in poor soils, they perform better if soil conditioner is used. It makes the difference between the look of bushland where plants are merely surviving and the look of a garden where plants are thriving.
Improving the soil does not mean changing its whole nature, however. If your soil is alkaline or acidic, it is best to accept the status quo and choose plants that prefer those conditions - which you already do when selecting exotics. Many garden centres will do a quick pH test if you take in a soil sample.
If you want to have some acid-loving species but your garden is alkaline, or vice versa, grow them in containers where you can easily create the right conditions. Use a fairly open potting mix for container cultivation. It is worth finding out the history of your block. For example, in a new subdivision that has been built where a market garden once stood, the soil may have high levels of phosphorus, which some Australian genera cannot tolerate - although plenty of others can.
Staff members of nurseries or garden centres that specialise in Australian plants will be able to recommend genera suitable for your soil, including those that tolerate phosphorus.