My case is slightly different to yours, but the experience might be useful.
Mr GG and I married after lodging our 136 visa application. We'd already lodged stat decs as part of the main body of our application (info re this at the link below), stating that we did not intend to marry at any point in future. When our plans changed, we just emailed our case officer to advise the change in plans and that we intended to marry. After the wedding, I kept my maiden name (and still don't plan to change it!) - like you say, for ease and to expedite the application. It wasn't an issue for DIMIA.
The website below states:
If you are legally married, you must include a certified copy of your marriage certificate issued by an official registry office.
Whether you are married or in a de facto (common law) relationship, you must include evidence that your relationship is genuine and continuing and you have a commitment to a shared life together. This evidence may include, but is not limited to the following:
history of your relationship evidence to support your relationship.
It also includes some useful suggestions as to how to structure your stat dec and evidence to support it.
HTH, GG
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