It's not "just living together", it's "living together in a marriage-like relationship". They want to see that you're basically living together like husband and wife but without the marriage certificate. So they want to see things like large joint purchases, having each other on life insurance or medical insurance policies, joint car or home insurance, and so on, and so forth. A girl and a guy can live together for a year and can have a joint lease and joint accounts, but none of the other stuff which just makes them roommates and not a couple. (I had this with a friend I lived with before Husband and I moved in together.)
My husband and I applied as common-law before we got married. We included our joint lease, our joint bank and utility accounts, a screenshot of him listed as my next of kin at work and vice versa, a statement from his medical and dental insurance listing me as his dependent, our joint house insurance policy, as well as the other stuff they look for like pictures, evidence of trips, evidence of communication, and so on. In addition, we also included details of the wedding we were planning like booking the registrar and the hotel and so on.
So it's important to differentiate between just living together, and living together like a married couple.