This is an interesting error. It's struck on a planchet that has thinned on one edge. Almost like part of the planchet just fell off before it was struck. But the straight line isn't like any planchet peel I've ever seen.
(http://www.australian-coins.net/gallery/albums/userpics/10001/australia-1964-1d-thin-planchet.jpg)
looks like someone's been into your chisels :)
Very cool.. Underweight?