The rule will always be the allowance for the marketing carrier of the first segment. If that marketing carrier chooses to apply IATA's MSC policy, then the MSC policy will apply. However, no domestic carrier applies the MSC policy, so we can rule that out if OP is on one ticket.

If OP is on two tickets, none of this matters as the DL policy will apply to SAN-LAX and the QF policy to LAX-MEL. If OP chooses to purchase an AA ticket, the AA policy will apply to SAN-LAX.

All of this points to the value of people providing a full set of facts. All OP need do is return here with his specific itinerary(ies) and whether it(they) is(are) on one or more ticket(s).