The crew donned oxygen and descended to 20,000 ft. The First Officer was subsequently incapacitated and the Captain descended to 8,000 ft and diverted the aircraft to Canberra.
I don’t know how accurate the above is if we can believe the data from FR24.
According to FR24 at 1325 the aircraft was at FL260 ( which it was cruising at) and at 1326 at FL210, 1327 FL180, 1328 FL126 and at 1329 10,000ft.
That seems like an emergency descent without pause from cruise altitude, by 1331 they were level at 8000ft.