Hose tap washer.
With the announcement of the proposed over water usage charges, plumbing maintenance will become more cost effective.
I get called out to leaking hose taps on a regular basis. A common problem is the washer punching through the tap seating. The seating is a raised piece of brass that the washer seals on, to stop the water flow. Unfortunately rubber will always lose against metal, however, it is normally a quick fix.
If your not sure then give me a call. Meeting your plumbing needs.