History of Australian ICAO designators

[Jason Carruthers]

G'day all I just reading a couple of articles about OJA's record delivery flight in August 1989. On the attached flight plan Australia seemed to be using ICAO codes beginning with "A" at the time ie. ASSY for Sydney and AMML for Melbourne. I just wondering of anyone can shed any light as to when we changed to the currently used ICAO codes beginning with"Y"?

Also unrelated to the above question when did we close the the SY and PH FIRs?

http://www.aussieairliners.org/b-747...hojastory.html

I can't seem to find any information with a google search.

Thanks,