Probably the most important part of any garden is the soil. In a bush garden there is probably not much you can do to change it but there are a few aspects that could be thought about.
Much of the problems caused in the bush are caused by the enrichment of the soil. Nutrients have been added by all sorts of means. There were a couple of rubbish dumps at Wirrapunga which still cause us problems. Sometimes fertilisers have been added to improve pastures. Runoff from roads contain all sorts of chemicals. etc.
The best way to remove nutrients is simply by plant material removal. High nutrient levels normally mean high weed levels. Removal of the weeds will reduce nutrient levels.
But there are two other aspects of soil I wish to look at here. There is no doubt that one of the best ways to introduce weeds to your block is by bringing soil, rich in weed seeds, onto your block. I noticed a grader parked on the side of the road recently on one of my walks. I presume it was going to grade someone's access or something. Anyway it had the name and address of the owner on it. It was from the other side of Adelaide. I also noticed the pile of soil it was carrying. There had been no attempt made to clean it in any way. I shuddered to think of the weeds it was about to introduce to the block it was about to work on.
The other thing I notice is the weeds bought in when seedlings are planted which have come from some backyard gardener or irresponsible nursery. In particular sour sobs seem to have been introduced to nearly every garden in Adelaide in this way.
However it is important to understand that everything that is not good can lead to something that is good. One of the best ways to introduce wanted indigenous plants to your bush garden is by bring in a bit of soil that contains the wanted seeds. If there is a bit of bushland that is basically free from weeds and contains some plants you would like then simply scrape up a handful of soil, put it in your pocket and spread it somewhere in your bush garden where you can keep your eye on it.
You will need to make sure that you have not introduced any nasties. If you do get something new, that you want, you will then need to encourage it to spread.