Done (I think).
http://www.australianwritersforum.org.au now redirects to /forum; search engine records have been updated so the hit on SEO for this change should be zero.
In addition, all of the non-phpBB pages have been removed. Using old links to any of these will now result in 404 errors. There will be a SEO hit due to the loss of this content.
There is still at least one link in the forum structure that points to one of the now-deleted pages ('Rules'). I didn't remove this, to prompt Those Who May Follow to recreate the subject matter elsewhere. There may be other such links lurking; my scan was superficial.