I'm pretty sure it's just illegal to have on your person without lawful reason. Self defence emphasised as not being a lawful reason. I live in Sydney and I own a large fixed blade, a small neck knife and a few folders, with a new one coming my way soon. I believe it's legal to own and have in your house, but that's it. Also anything that can be used as a weapon seems to also get you into trouble. A policewoman came to my school a few years back and told us that scissors could get you into trouble if the circumstances were suspicious, eg. a teenager with scissors in a street at night. I think it's just up to the discretion of the police to determine if you get charged. As you said, it is very gray and I just don't risk carrying because of the way I dress and my age. I know I don't have specifics, but I hope that helps a little!
