Where your getting the ply from? If not theres sure to be some boatbuilders in Sydney that work with timber. If you've got a router theres a couple of easy ways to do it. Have you googled scarfing, theres heaps of jigs you can make from all different tools, routers, circular saws and by hand of course.

If you get your sheets scarfed by someone else, they'll probably want to join it to, because they prefer not to risk the damage that can happen when moving it from a to b.

Regards