Virgin Australia's compensation policy for a flight delay of greater than four hours is to provide a credit to the full value of the affected fare. In this case, that would be however much American Express has paid Virgin for this fare. Ulimately, that comes down to what the commercial arrangements between Amex and Virgin are for these fares. See here for reference:
http://www.virginaustralia.com/Terms...nsationPolicy/
Personally, I find the compansation to be quite generous.