List of reputable and recommended RMAs that frequent this forum are here:
http://www.australiaforum.com/visas-immigration/252474-registered-migration-agents.html
These ones are all legit and incredibly knowledgeable. No need to be based in Melbourne as this type of stuff is typically handled electronically via email and Skype.
Up to you on whether you should use one or not. If you have a lot of evidence of your de facto relationship over those 4 years, then it could be relatively easy. The way I like to give it as an example is this: Changing your own car oil is an incredibly simple process, but most people just prefer to pay someone else to do it. There is a bit of comfort and confidence in knowing that a professional is helping you submit a thorough and well organized application following all the rules and guidelines that you may not even be aware of.
In your case, if you have de facto evidence for the three years prior to your application, you can have the two year wait period between application date and PR eligibility waived, meaning you could go straight to PR when they assess your temporary 820 visa. An RMA would be able to help you present your application well with supporting evidence with this in mind. It can significantly cut the process down for you as current wait times AFTER PR eligibility are 1-2 years, making the entire partner visa process a 3-4 year thing whereas depending on how long they take to assess your 820, you could be done with the whole thing in about a year.
Edit: My partner and I did the application ourselves. We first started researching partner visas in July of one year and didn't end up applying until December, and even then we learned quite a few things afterwards that we wish we had known before we applied. A lot of people on this forum take several months before applying to really understand the entire process and learn the ins and outs that aren't obviously listed on the immigration website, and some also take weeks to months to gather evidence and organize it properly. So if you're willing to put in the effort to do your homework, you can do it yourself. But again, sometimes it's just easier to have a professional guide you and know that you did everything correctly. Immigration isn't very forgiving of mistakes!