Hi Bonus
They would use much the same criteria as they do for de facto Partners in the Family Stream of visas I would imagine. Please see here:
Australian Immigration Fact Sheet 35. One-Year Relationship Requirement
You should have a chat on the phone with a Registered Migration Agent I would suggest, because they are the people best-placed to tell you - based on their experience with other applicants - whether the amount and types of of evidence you have currently would be sufficient or not.
By using a WHMV though you can sidestep any potential queries at this stage and give yourselves enough time spent living together etc to shut DIAC up. If DIAC were to start crawling over the domestic side of a joint application with a fine toothcomb, it could cause serious delay and it could also lead to them threatening to refuse your own 457 application unless your girlfriend is withdrawn from it.
Why buy yourself a potential headache when there is no need to? A WHMV only costs $190, they are usually processed abnd granted in less than a week, and it is very simple to apply for it on line via the link in my previous post.
Also, it would be better if your girlfriend were in Oz in her own right so that in the event that you split up after arrival in Oz, she would be able to remain in Oz independently of you.
Best wishes
Gill