I thought I would update this post as I have spoken with Pass the Parcel this morning and they do not deliver to Great Barrier Island. The website is accepting the address in error. If I had sent the parcel it would have been held and returned to me.

Mine disappeared too.

Thanks Gary. Yes, the rural delivery/out of area surcharge is showing for PTP. I have emailed PTP to confirm it is fine to send, but have had no reply. I don't use the BAC usually either, and I think the software has just tried to find a close match to my own delivery price and so offered up the non-rural option. With NZ Post's website all the options had the RD charge applied.