Many thanks Chris - after playing around with Awardnexus, I rechecked availability of award flights via BA OneWorld and gave it another shot with Qantas call centre.  This time seamless!  Locked in 4 business class fares each way in January.  A mate of mine just did the same thing,...

I checked Melbourne Tokyo availability via the above BA/Avios method fo Jan 2018 and noted quite a bit of availability (e.g. quite a lot of days had 4 seats available in various classes).  I then called Qantas customer service and they told me the seats were not available for awards bookings...