I think that can be caused only by the square pieces that hold the bits to be different height, and/or the bits themselves to not be accurate. It should be correctable without any special tool.
Check that the square parts mentioned above are assembled correctly, that is they are inserted on the same turn of the threaded bar. Also, avoid tightening their knobs, so they can align better.
I'm guessing Australian slang for eBay. Then the eBay is the place where you can pick up almost everything sooner or later if you're patient and know how to search for things or you just stumble across it by accident. A lot of times unfortunately the sellers sell things in assortments and won't describe, Or don't describe what they're selling. Sometimes that's outstanding for the buyer but it's a pain in the ass If you're looking for scrap movements and they don't have a description.