Try the Research Centre at the Australian War Memorial (AWM):
https://www.awm.gov.au/contact/Click on "Ask a question" on the Research Centre line on the AWM Contacts page, and again on "Ask a question" when the RefTracker page displays. This will bring up a form for you to fill in. I've found the Research Centre to be pretty helpful. They probably have the dress regulations for that time and quite possibly some actual uniforms.
You could also ask the Army History Unit (AHU):
ahu.enquiries@defence.gov.auAHU isn't set up for personal research, but most likely can direct you to somewhere/someone with the answer. One possibility may be in the Army's extensive museum network. A good candidate would be the Infantry Museum at Singleton (02) 6575 0257.
EDIT. The example portrait is of a British Officer in 1924. Are you specifically interested in Australian uniforms or both British and Dominion uniforms?