Fifita was arrested on Wednesday night and a court on Thursday heard he wrote the message because he considered it amusing, Stuff.co.nz reported.
But his lawyer says he's still in custody because Air New Zealand won't allow him back on one of their planes.
'The captain came out and said the guy had said that he had a bomb on board, ' an Australian woman who was a passenger on the flight claimed.
Police were called just before 9pm after airline staff were told about the comment by another passenger.
The teenager was arrested at Auckland Airport and charged with breaching the Civil Aviation Act, according to Immigration New Zealand.
On Thursday, Manukau District Court heard Fifita was travelling home to Sydney from Tonga, transiting at Auckland, when he made the false threat.
The flight was cancelled.
An Air New Zealand flight to Sydney was cancelled last night, as well as the ensuing flight today from Sydney to Queenstown.
After the detection of the "threatening comment", the plane, which was taxiing to the runway, returned to the gate as police and firefighters swarmed the airport.