Whilst it may seem strange what Qantas are doing it does make sense. They have asked for a wage freeze with other unions. Most have accepted some form of freeze.
These guys have been on overtime bans since September and it is strating to become unsustainable for QF's maintenance scheduling so they are trying to find a solution. A negotiated solution was voted down so I guess they are playing hard ball.
The precedent they are using is a valid one. These guys are refusing to do overtime where there is a convention of normally doing some overtime. There have been several cases sucessfully argued by companies in the IRC that an employee has to do a reasonable amount of overtime. This could get quite ugly as QF appear to be very determined this time around.