My partner and I went through the same process with an 820 visa although didn't bother with an agent. You do have to be careful with this kind of thing, don't just assume that the migration agent is crazy. A friend of ours visa was cancelled and she had to leave the country because DIAC somehow found out her relationship ended 8 weeks before her visa became permanent!
Evidence of your relationship doesn't have to be official correspondence from companies. Do you contact each other throughout the day? text messages and phone calls? emails and facebook messages? Print them out and use it as evidence, send phone bills highlighting your calls and texts to each other, you can even print off emails and things like that. Have you taken a flight anywhere in OZ during your stay here? Find the booking email and use that - it will have your names on it. Also, if there are any photos of the both of you with an obviously Australian background you can use that too.
Good Luck!