I beg to differ, as the process is long and complicated if you are not familar with the process. An agent will help you through short cuts (though not always, case-by-case basis) and cut down time reading and figuring out how to provide and fulfill the process requirement. It is more so if the applicant is applying in a family which requires to submit countless of documents or an off shore applicant. The DIAC will also demand you to resubmit documents if deem unsatifactory.

The conclusion is you'll need an agent if:

1. English is not your first language, you'll need to fully understand the terms in the forms so that you furnish them the correct/required information. Else you risk delays, given a run around or even rejection.
2. No contact, relatives, or friends residing in Australia as your reference (depending on the type of application)
3. Off shore applicant (meaning applying from outside Australia)

Some years back when I was looking the agent fees in Malaysia were roughly RM7-8k, independent of whether you were applying alone or with a family.

Even if one fully understands the forms and instructions given, you still lack of the tips & shortcuts of getting through and fast, which I think it all matters at the end of the day. Knowing someone who have gone through the application process is good, but remember all applications are vetted case-by-case basis so it might not apply to yourself.
You sounded like a permanant visa holder/citizen, mind sharing what sub-class did you apply for?