I'd call back and explain you just realised the SYD connection is very short and whether it'd be possible to go (say) an hour and a half or so earlier. If you are claiming GST refund before departing SYD, you must allow for 30 min in the queue - and the cut-off for processing any claim is T-30 min from departure - strictly enforced. You haven't a snowflakes chance of even making GST refund with 65 min connection which has to include includes the (free) connection bus from domestic to international as well as queue for immigration.