Excerpt from the relevant Australian Order, CAO 20.4.
8.4 Unless paragraph 8.5 applies, where a pressurised aircraft to which this subsection applies is operated above Flight Level 250, then at least 1 pilot seated at the controls of the aircraft must use supplemental oxygen at all times during which the aircraft is operated above Flight Level 250.
8.5 Paragraph 8.4 does not apply if an aircraft is equipped with a quick- donning type oxygen mask for the pilot or, if more than 1 pilot is required for the flight, each pilot. However, whenever the aircraft is operating above flight level 450, the pilot, or one of the pilots, seated at the controls of the aircraft must wear an oxygen mask that is properly fitted and supplying oxygen