Check the 'similar threads' section at the bottom of this page - there are a few threads there about it but unfortunately the bottom line is that it costs big money to replace it. I also remember reading somewhere that Holden don't stock new covers anymore so I'm not sure what you can do.
"If anyone disagrees with anything I say, I am quite prepared not only to retract it, but also to deny under oath that I ever said it." --T. Lehrer
"The secret of success is sincerity. Once you can fake that you've got it made." -- Jean Giraudoux