Okay - Regardless of which law we're talking about, one common element that is required to be classified as a defacto is that you must be "living together as a couple". So the first thing that needs to be established is whether or not you are actually living together. Only then can the question of a defacto relationship be looked at.
One way to determine if you are living together or not, is to establish where her "primary residence" is. So lets's go through a few things.
1. She lives with relatives when not at your place, so we now know that she doesn't have her own place. That means no ownership or rental agreement in her name. That's unfortunate, but certainly not a deal breaker.
2. Since there's no residence in her name, you need to look at what address does she use for "everyday living". So you look at the important things like driver's licence, car rego, mobile phone accounts, bank accounts, etc. What address is she using for all of those things? If she is using an address other than yours for everyday living, then this provides a very strong argument that your place is not her primary residence.
3. She doesn't have a key. Given that she is not living with you full time, not having a key means that when she is there, she is a guest, not an occupant. The simple fact is that she can't come and go as she pleases - she can only "visit" when you are home and you grant her access.
4. Another thing to look at - are you renting? If so, is your name the only one on the tenacy agreement and is the number of additional occupants zero? If so, then the tenancy agreement also indicates that she doesn't live there.
5. The reason I asked about other occupants having a key is because if anyone else did live with you and they all had their own key, then this would provide you with the argument that you only give keys to people who are living there - you didn't give her a key because she doesn't live there.
A lot of things need to be considered in order to determine if your place is her primary residence. What address she uses everyday is a big part of that, so is the fact that she's not there full time - and so is the fact that you have never given her a key.
If you can get enough evidence together to show that you are not living together, then you can't be in a defacto relationship.