Encourage her to get professional help - as much as you are her best friend it is unhealthy for you to take on the role of a psychologist.
Let her know that you are always there to talk (etc), and that you care. Force her to get out of the house and go shopping/see movies/etc, with you :) pull her arond to your house :)
Be assertive and honest about your feelings to her. You cant spend every ounce of time with her, and being with someone suffering from depression can be very psychologically draining and can effect your mood. So look after yourself, because if you get down too, the situation will only get worse. If and when you need 'me' time or whatever you call it, just let her know that you care about her and arent bored of her, that you just need some time by yourself :) this is because some people with depression tend to read into things too much. Goodluck with everything and I hope your friend starts to get better!
Author's response
I tried/am still trying to get her to see someone, but she keeps on saying that it doesn't help anything, she's been to therapists before and she says it just made everything worse... I can't exactly get her to come to my house as I live in Brisbane, I'm down here visiting for a while, but I'm staying with her, we just spent the last two days watching movies, surfing, painting and shopping, but she doesn't seem to be getting into it at all... she used to love painting so much, but now everything just turns out dark and depressing... I'm staying with her for the next five weeks, I just hope it's long enough to help her through this...