For you to be able to make an appointment with bupa, you need hap number. And, from memory, I think you get that by creating a health assessment application in your immi account. I did the same thing, and now that I have hit the 12 months mark I am regretting it. I should've done it few months after I lodge the 820 application.
But it is your personal choice if you want to do it before or after the lodgement of the application.