I think I would sand down to the void and around it then build it back up with layers of fibreglass cloth. You could try finding the top of the void then drill two holes halfway into the plywood and fill up through one hole with epoxy if your still not 100% cut a bit of plywood and epoxy it over the panel. You should be able to get marine ply in Australia there are a few boat builders here that point you in the right direction http://www.glen-l.com/phpBB2/index.php
Best of luck.