The plane was grounded on Saturday after Civil Aviation Safety Authority officials undertook a random drug and alcohol test of all crew, prior to their pre-flight checks.
The pilot who failed was stood down and has been suspended from all duties.
Singapore Airlines says he's returned to Singapore where a full investigation will be undertaken.
It's promising to work closely with Australian and Singaporean authorities to ensure they're given any information they require.
Singapore Airlines has apologised to those affected by the cancellation.