Following the additional info from Alex I have tried to do some more research on this subject. I am certain that Hughes transferred to the RAF on a ssc following graduating in Dec 1936. Given the undoubted evidence through photos of Clisby (killed in May 40) there is precedent for these chaps to retain RAAF uniform. In looking at wiki (a dubious source if used without correlation) today I see the comment ref Hughes 'Always a proud Australian, Hughes always wore the slightly darker blue uniform of the RAAF throughout his service with the RAF, but with RAF insignia.' This may explain why it appears in photos that he wears RAF wings.
The RAF ssc idea was agreed at the 1923 Imperial conference and commenced in 1926. A portion of each RAAF graduating course would transfer to the RAF to serve four (+one) years before return to rejoin the RAAF. This probably explains why they may be listed in some records as still RAAF. Aus and UK govts would split the costs. The first seven left Australia in Oct 1926 and the scheme continued to 1938. Many of these officers decided to remain in the RAF permanently eg Hugh Edwards VC, DSO, DFC, Don Bennett AOC PFF. In 1935 the UK Air Ministry requested extension to six years. About 20 members of the 1936 class went to the RAF. My reference states that up to Jul 1938 149 RAAF officers had been sent to the RAF on ssc following award of wings in Aust. I have reserached about 50 of these chaps (incl the whole 1936 course) in the past and they are listed in the RAF List.
Just found this link. Have a look at the photos of the three airmen near the top under the heading Aldergrove Jan 38.http://users.cyberone.com.au/clardo/raaf_personnel.html
These chaps graduated in the 1936 Pt Cook course and are still wearing their RAAF uniforms in RAF service 1938.
Regards - Mark