Anyway, Radio NZ has this huge sound file, and something they unearthed and played on the afternoon show solved this mystery. If you listen to my interview/guest spot/thing, the sound file occurs 11:30 into the aired clip NZ Retro - Ghosts. (If you want to listen to the entire show from 20th July, 2015, go ahead. There's also an interesting account concerning Larnach's Castle - a famously haunted location in Dunedin, New Zealand.)
So, case solved! This information was broadcast at the time for all to hear, and in the 1960s people actually listened to the radio so there wasn't much chance of it going unheard.
One wonders how often this happens in the world of the paranormal. How many famous, well-documented cases were actually solved but the crucial, revealing information was later ignored, skirted around, or merely hinted at, for the sake of preserving a good story?