This is a common mistake FF make! If there are no space+/PE seats left for selection you are best to not select a seat. Most people will select a worse seat or in your case upgrade, however if you do not request any seat at around T-48 hours to T-24 hours, or sometimes earlier, you will almost certainly (as an Elite) be given one of the PE/space+ seats that are blocked. You will notice a seat is usually added to the booking before you check in that you wouldn't usually be able to select.

I have done this many times crossing the Tasman, in the past more seats would be available for selection at T-48 hours online, however more recently these seats now stay blocked until check in, so best to wait and let NZ do it for you.

Next time try this instead of putting up with a worse seat or paying more for WD!