Will there eventually be an R 18 rating for video games in Australia

no, the present classifications are g,pg,m,ma and some games which are too violent are banned.
but there might be a chance that we can stop the ban
if this is not enough information for you check out these websites
http://en.wikipedia.org/wiki/Attorney-General_of_Australia
http://www.inmycommunity.com.au/news-and-views/local-news/R-rated-games-call/7551276/
http://www.globalpost.com/dispatch/asia/100111/video-games-ban-Australia
http://en.wikipedia.org/wiki/List_of_banned_video_games#Australia