You can consult a migration agent if you want a definitive answer (I'm not an agent) but my opinion is that you need to provide it right away. Like I said, omitting information to DIAC is a big deal. You risk having your visa rejected, or if they find out later, revoked once you're in Australia and already settled in to your new life. Don't risk it.
Original Nationality: US
Visa #1: PMV (300) through Washington, D.C
Applied: April 2013.
Visa Granted: January 2014.
Visa #2: Subclass 820 (From PMV).
Applied: End of April 2014.
Visa Granted: Early July 2014.
Visa #3: 801 (PR)
Eligibility Date: End of April 2016 (Applied a month prior).
PR Granted: Early April 2017.