I have a photocopy (kindly sent to me by Ron Lee) of a similar cover to London - same handstamp (also signed Joan McLennan), Gowrie label on the front and postmarked at Hobart on 13 March 1941.
In their excellent book on WA Forces, POW and censor mail, Haynes& Pope (WASG 1997) discuss the 'Control of Overseas Postal Communications Order (No. 1). At page 457 they say "Governors, Ministers of State and diplomatic representatives were specifically exempted from the Order as was mail from POW in Australia and to POW outside Australia, both of which came under other regulations. This exemption gave rise to the rare gubernatorial 'Privileged Mail' cachet so far recorded only for the Governor General, and the Governors of Queensland, Tasmania and South Australia between 21 September 1940 and 16 August 1943. (ACSSGB, April, May, July, August 1969). The operative effect of this cachet was equivalent to 'Not Opened by Censor'. "