I don't think it's so much a mattress as a simple seat topper to smooth out the seat surface for sleeping. This was offered when I flew MEL-SIN last Christmas; you may have to ask the crew if it's not proactively offered.
I've been flying QF J 333 to CGK and SIN at least once or twice a year since 2010, was never offered a mattress/duvet/seat topper/smoothener/whatever even once, though my last flight was in May 2013. Neither did I see anyone else in the cabin ever receive it. Perhaps this is a recent enhancement (in the true sense of the word lol) that took place in the last year or so. Maybe someone else can confirm.