I had the same thing on a BA JFK-LHR flight for my DONE4. My TA explained that BA restrict the quantity of seats available to different markets for some flights. So what I am seeing on ITN is the availability for sale in the USA market, and that may differ from what your TA in NZ or or my TA in Australia is able to sell.

However, if ITN is showing D9, then get onto the waitlist and it is very likely to clear. Mine did after being on the waitlist for about a week (this was about a month before that flight date). It seems BA will review the waitlist and teh chances of clearing are very good if there is still a lot of D available on ITN for the USA market.

After my waitlist had cleared, I eneded up changing to a different flight to cater for a changed connection at LHR. But then even later wanted to go back to the originally waitlisted flight due to some other changes, and I was able to be confirmed straight onto it, so it seems BA had opened up the D for Australian sales and left it there even after I changed.

I had not come across that before, and any time ITN showed D available I was able to book it from Australia. So I was at a loss as to why I had to be waitlisted. I had to push the TA for an explanation, and she said "Well, its a bit complicated, but it works like this ...". I am just passing on her comments and she has been pretty good/accurate with info in the past - although I have had to clarify a couple of xONEx rules for her .