There isn't much reason that would be accepted beyond "for work". A friend of mine had a box cutter on him on a train and got pulled up by police. He explained it was for work. They called his boss to confirm before letting him go. Also, NSW seems to have the least amount of tolerance. There is basically no leancy unless you get a nice cop... I was in a friends car once and we got pulled up by police. They searched his car and found a swish army knife. He told us to get out of his car, made us put our hands on the his police car bonnet while he did paper work. My friend was that close to being charged for it, but the police believed him in the end, that it wasn't a weapon. That was after much consideration and seeing his name encraved on it and believing him that it was just a sendimental present. I also used to carry a screwdriver in my car before one day having my car searched. The police found the screwdriver, questioned and hassled me about it. But having found nothing else, and I was cooportative during it all, they let it slip.