The main reason these tunics, and other items were cut down was generally because of the lack of supplies. This is especially the case in the WAAAF and ground trades, and I have many personal testimonies from former WAAAFs in particular who told me that aside from issue tunics being cut down to fit, in the early days mens overalls and also service dress tunics were tailored and cut down as womens simply werent available. It was also considered good practise for the girls. As such, many early dated WAAAF uniforms are actually cut down and retailored mens issue.
Ive had three of these BD types, cut down from mens SD tunics. One former WAAAF told me that as the BD began to be issued supplies were again short, and initially issued to aircrew, so few got them in terms of WAAAFs and ground trades. As a result they made their own from SD tunics, which was seemingly either approved, or officially ordered. It was certainly commonplace amongst groundtrades and WAAAFs, as seemingly the aircrews were the first issued with BD on a large scale.
Mark, you mention the collar of the tunic, and I agree that its more like the BD style and not the SD. But on the other hand, if you are going to remove half a tunic, and retailor the skirt, the lining, the belt, and maybe the fit, then reshaping the collar to make it more 'BD like' would be an easy part of the process, certainly not a difficult or lengthy job. No one would butcher a BD for the collar, to convert a service dress to a BD, but the removal of the whole skirt would provide plenty of excess material. You can see on the closeup that a long strip of material, above the natural step, has been inserted on the collar, and I would think a new collar from the excess material.
You mention the metal disc buttons being replaced by standard black RAAF issue. This is not uncommon to any nation. I have a number of photos showing RAAF, RAF etc (including a well known RAAF Pilot) who have replaced their BD buttons with issue. In the case of the RAF especially there were the compass buttons made specifically for BD epaulette wear.
The RAAF Dental Branch 'BD' I have, which has been cut down from a mans 1942 dated SD tunic has had all issue buttons removed and replaced with plain black plastic, which is odd considering it must have been issued with regular RAAF buttons when a service dress. It was certainly worn as such, and I have the same persons tan summer dress tunic, late war with Corporal rank (the blue BD has no rank).