On New Years Eve, 1976, 36-year-old Heidi Charles vanished without a trace. True crime writer Scott Bainbridge explains how this baffling case has been mixed up with other high-profile disappearances.

On New Years' Eve 1976, 36-year-old Heidi Charles was dropped off in central Rotorua to do some shopping.

She arranged to meet her husband, Robert, and two sons an hour later at Fenton Park. But she never turned up.

It was one of the busiest days of the year in the holiday hotspot, but no one has come forward with information about what might have happened to Charles.

There were two possible sightings of her, one in a clothing store shortly before she was due to meet her family and another on the outskirts of town.

Police went to extreme lengths to try to figure out what happened to her, including throwing a sheep carcass into a hot pool.

Go to this episode on rnz.co.nz for more details