On 11 January 2007, an Air New Zealand Airbus A320 which had just departed Sydney Australia for Auckland, New Zealand on the first flight of the day was observed to have turned onto a heading contrary to the ATC-issued radar heading. When so advised by ATC, the crew checked the aircraft compasses and found that they were reading approximately 40 degrees off the correct heading and that a ˜GPS PRIMARY LOST™ message had appeared on the MFD and the NDs.