I secured both seats in F at the same time and was approx 8 months in advance each time .
I booked mine not via BA and Avios but with AAdvantage ... but I think that you should be able to access the same via BA and Avios - though their online booking facility doesn't seem to be able to book the trans-Tasman sectors but I think if you call BA they can sort them out ...
Hope that's helpful !