You sure can if you wish. You are also free to defend yourself in court, or to take on a matter in the AAT by yourself, if you’re confident. However in these cases you do so entirely at your own risk as the critical risk is losing the Australian Government related fees (visa application charge etc), which can be substantial – particularly if you make a mistake or the application is somehow deficient.
In most cases we manage to stop a client from doing something that would have caused them problems with their applications, maybe even resulting in their visa refusal. However if you do-it-yourself, you are missing out on the services of experienced RMA’s who have been through this process many times before.
There is a key question to be answered: What price do you put on your and you family’s future, and would you do it yourself if this were an important medical procedure?