The partner migration booklet is out of date. Best to follow the instructions on the immigration website for your visa.
Nothing gets posted in, nothing gets uploaded as a certified copy. Clear color scans of the originals are what is required.
Even the F888 is not a certified copy; only the signatures are certified as those of the authors.
The booklet checklist was based on paper applications which no longer exist as online is the only way to apply now. The only reason paper applications required certified copies was because you wouldn't post them your original birth certificate or passports or any other important paperwork, so you'd send copies that were certified as true copies of the originals. Obviously this is irrelevant now. So upload clear color scans of original documents, including police checks (don't post these).
As for proof, have you talked about your plans to move and live with your partner via messages with your partner, family, or friends? That's one way. Another is to explain your plans once you get to Oz in your statement.