I had the same problem where I was lead to agency so we did bookings directly on the airline's website by max number of pax 7 adults and 3 kids, put the price on hold and before paying it all we made another reservation for 3 more adults in our group. So we did not have group booking rather just two reservations for 13 people including 3 kids and we controlled the price perfectly. Honestly, rocketing price for LON-SYD isn't surprising.