When I flew Qantas in November as a One World Sapphire, they sent me to the Cathay lounge. Back in May they sent me to Air France. Several travel blogs covered the Cathay agreement with Qantas just this last week as if it was something new, but it had been in place for a few months. Any lounge reserves the right to restrict capacity. Maybe you're running up against that with anticipated evening departures, which could include Qantas flights to both SYD and MEL as well as a Cathay flight to HKG. Whose One World card are you trying to enter under?
BA's current agreement, during remodeling, is not with Cathay.
Note: Our San Francisco Lounge is closed while we work to bring you an entirely new lounge experience. You're invited to use the China Airlines Dynasty Lounge in the interim.
https://www.britishairways.com/en-us...siness-lounges