Sometimes QF will release a seat upon request by a Platinum FF member. But it depends completely on whether they think they can sell the seat later. Otherwise you have to wait until about a week before the flight until they review the Z availability. However, anyone with a paid business class seat and waitlisted for an upgrade will get first look any seats released into the award Z class.
First class award seats are scarce and generally are taken very quickly when they become available.
A friend booked two first class tickets to Hong Kong the other day, and when someone else stated they wated to come along, he just logged back into the website and booked another. Three on one flight without doing anything special.
I should say that he's a plat FF member... but I don't believe this came into play at all.