Hi Saozita,
Having BMI over certain level does not necessarily mean you will
not meet the health requirement.
If you are assessed as being obese, some exploration of related medical conditions (which can exist or be emerging) will be made.
These include diabetes, heart disease, hypertension, arthritis, reproduction or .. something more serious.
Conditions I just listed may mean that you are assessed as not meeting the health requirement, but if there is no existing or potential condition, you might be fine.
They will take your blood and do additional tests for sure I believe.
Australian Government would asses your current and potential healthcare cover cost over the next several years (I believe its 5, but don't quote me on it please, I'm not a migration agent

).
Even if you don't meet the criteria for any reason (which I hope won't be the case), there are ways of waving this obstacle, but you would have to consult with a lawyer about it (there are migration lawyers who make a living out of this type of cases).
Not what's your current BMI, but maybe if your medical is not booked for another couple or a few months, you can try and work on it.
Good luck and let us know how it goes!