There are other determining factors that go into getting the visa. I would look into those first to make sure you meet the rest of the criteria before stressing about his criminal charges.What is the likelihood of us getting the visa?
You have to show evidence of a one year de facto relationship (I believe, unless your relationship is registered in Australia, but I have been wrong before, mostly of late) and from your own post it says you've only been de facto for 4 months.My partner and I have been living together in the UK since the very beginning of June 2017 and have been in a relationship since January 2016.
You're in the right place. While people may not be able to give you the definitive answer you are looking for, there are people here who can help. A consult with a RMA would be a great place to start.
Here are a list of the ones who frequent these parts.
Hope this helps and good luck.