San Francisco -> Fiji -> Brisbane -> Melbourne, Australian Travel Declaration

This Australia government webpage describes the information required to complete the ATD:

When filling out the ATD, you will need to:

• provide your contact details, including a contact phone number in Australia
• make a legally binding declaration in relation to your vaccination status
• make a declaration regarding your last 14 days travel history
• make a declaration that you are aware of the quarantine and testing requirements that apply when coming to Australia and in the state or territory you are landing, and the penalties for non-compliance.

The third bullet point will include all your Australia-bound flights. Thus, the answer to your first question is not "this flight" or "that flight," but both flights.

You will arrive into Australia at Brisbane (in Queensland). The flight from Brisbane to Melbourne (in Victoria) is domestic, both departure and arrival airports being in Australia. To make sure that all passengers arriving in Brisbane from an overseas airport (such as Fiji) pass through Australian Immigration and Customs upon arrival in Australia, Immigration and Customs checks will be done at Brisbane. That you will not leave the Brisbane Airport is irrelevant.

Thus, the answer to your second question is you should complete the Queensland declaration form. I am unfamiliar with the existence or content of a Victoria arrival form. It may be that one flying from Brisbane to Melbourne (as you will) is required to fill out such a form as well.