Have you tried phoning BA Sydney res or QF res if sector on QF? They won't be able to refund the miles but can cancel the reservation/release the space on the flight and you can sort out the financial bits with Exec Club later. I had a similar problem recently and the QF ticket desk at SYD A/p cancelled a BA Miles reservation for me 25 hrs before STD as unable to do on MMB. QF put a note on the res history that passenger had requested cancellation. I then phoned BA in UK when open & explained - Avios duly refunded.