The pilot reported "Leaving Flight Level 370 for Flight Level 350" when the aircraft was about 320 kilometres to the north of Perth. The aircraft had not been cleared by Air Traffic Control to leave Flight Level 370 on descent. Investigation indicated that the pilot was only requesting a clearance to descend and his use of non standard radio transmission terminology gave the impression that the aircraft had already commenced descent. The aircraft remained at Flight Level 370 until cleared to descend by Air Traffic Control. There was no DEPARTURE from standard operational practice. This occurrence was not considered to be an air safety incident.