Some interesting developments. Qantas came back via Twitter to advise "it seems that there's been an oversight from our end"I think this may be my situation. Started a conversation on Twitter to convert a current surface sector in my OWA to an award flight thanks to new availability on AA recently appearing. After waiting several hours for a response I was advised that flight could be added to my itinerary for the 5000 points change fee and additional taxes of $1,777.85 per person!!
I asked them to double check and they came back with the same response so I decided to not proceed. (I actually booked a separate partner award flight for the same sector some months ago so am covered anyway)
I decided to do a dummy booking on the AA website for the same flight and the taxes came to a total of US$48.94 (approx. $70) per person. Furthermore to pay full price for a business class seat on the same AA flight would have cost US$991 (AU$1432 or $345 less than the additional taxes)
I seems therefore that Qantas has recalculated taxes for my entire 10 flight itinerary - which was ticketed some months ago - and it appears some operator fees have significantly increased.
What confuses me though is that about a week ago I managed to get one flight in our itinerary changed to exactly 1 hour later to meet MCT requirements and I wasn't charged any change fee or additional taxes yet the itinerary was reticketed.
By the way it appears that the staff who respond to Twitter messages (and I assume Facebook Messenger) appear to be quite competent as compared to O/S based staff - just very slow to respond.
They are now calculating the tax difference as AUD156.93 per person - a lot more like it but still more than I get by dummy booking on AA or ITA Matrix.