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The pair have been pen pals since their early years at school, when Mrs de Geest lived in South Otago and Ms Erickson in Selkirk, a town in Manitoba, Canada.
The first letter was written in 1956 and over that time, the pair have shared virtually all of their life experiences, including marriages, raising families, deaths and everything in between, such as music and sport.
They used to write six to eight page letters to each other once a month, but now do so twice a year, including Christmas.
On Tuesday, they met for the first time when Mrs de Geest picked up Ms Erickson in Christchurch after she arrived in the city to start a 16-day coach tour of the country, one of many overseas trips she has embarked on since her husband died about eight years ago.
The pair spent the day together and caught up briefly in Oamaru on Thursday, when Ms Erickson's tour hit town.
She said meeting Mrs de Geest after knowing her for so long was a special moment.
''It's amazing, I don't even know how to express my feelings. It's exciting and amazing and it's something that I have wanted to do for so many years.''
Mrs de Geest felt the same way.
''It's just great. After writing all of those times ... it's special. I guess it was strange, because I feel like I know her family.
''We wrote about daily life, world news, school, family and sporting activities and exchanged lots of photos and souvenirs.''
She said the pair would continue to write to one another by hand, as they always have, which meant no phone calls, emails or text messages.