Strange light changing size & making zig-zag movements.We were standing outside in our backyard observing the stars as it was a perfectly clear warm summers night. We were facing south, and in the background are hills and Mt Wainui and in the foreground are trees to our left and to our right. We noticed a point of light in the sky facing due south at about an angle of 55 degrees. It was the behaviour of this 'point of light' that caught my eye. When I first observed it, it was about the size of the star Canopus. It was only seconds after this that it rapidly expanded in size and brightness to the size of about my fist held straight out in front of me and the suddenly shrunk rapidly back to a small point of light, smaller then what it was to begin with. It then proceeded to peform unbelievable maneuvers, climbing vertically and down again, turning at right angles and moving in a zig-zag fashion at incredible speeds as if intelligently controlled. It eventually disappeared moving at high speed in a SSE direction. I love observing the sky and have been stargazing for years so I can quite confidently say that it was not a plane, helicopter, weather balloon, satellite, planet, shooting star or meteorite. There is a flight path that passes over us from Wellington and a small aerodrome about a kilometer west of us so I'm 100% sure it wasn't a plane, as all planes that pass over are easily identifiable. Although it was an outrageous sight to see, I must admit it left me a little unnerved.