[xwiki-users] Wiki setup as authenticated user

I'm trying to set up a XWiki site using container (tomcat) authentication. The authentication pat is working fine, but I'm having any issue when initialising the site for the first time. Normally (without authentication) xwiki spots that the wiki doesn't exist, and creates it (presumably using some magical user it creates). But when I have authentication the initial wiki creation step does not happen and you go straight to a "page can't be found" message, even if the authenticated use is part of the XWikiAdminGroup. Is there any solution to this, or must the wiki be created before any authentication is put in place? Thanks Tim