The affair wasn't revealed until Cecilia Brooks, pregnant with her sixth child, was told she had blood poisoning.
Her husband Bert Brooks, a building contractor, had arrived at their Te Ore Ore home near Masterton to find her weeping.
She told him that if she died he would find a letter behind the washstand.
How long Bert waited before reading the letter isn't clear in the Herald report of his successful action in the Supreme Court to divorce her on grounds of adultery.
What is clear is that the letter contained a confession of her affair with wealthy Wairarapa sheep farmer Donald McKenzie.
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It was 1919 and divorce was more complicated than it generally now is and involved a court case.
The Herald said the confession letter "indicated that familiarity between the parties began in Auckland in 1915, and was continued in the following year, when the petitioner [Bert] and his wife returned to the Wairarapa".
"A child was born in October, 1916, and McKenzie himself claimed paternity of this child. He treated this child as his own, and did the same with the child born in January last ."
The Brooks had married in 1901 at Stratford. Bert told the court they had four children together.
Cecilia said she had her fifth and sixth children in October 1916 and January 1919 and that both were McKenzie's. She wanted their paternity legally determined by the court so any benefit of the court case might accrue to them.
McKenzie denied that he had ever "misconducted himself" - to use the court case's phrase - with Cecilia Brooks. His wife had died of influenza in November 1918 during the epidemic.
A report in New Zealand Truth, far more detailed than those in other papers, described McKenzie and Cecilia Brooks' liaisons, which would sometimes involve a drink or a drive in his car.
A parade of neighbours and housekeepers told of the couple's visits and a night spent together in the one bedroom. Once, when Brooks' husband was away, Cecilia stayed with her lover for three days, while at other times she would send her children to the neighbours when McKenzie visited.
A nod's as good as a wink to a blind horse!
Nellie Cameron, a housekeeper at McKenzie's, said after Brooks had been out with McKenzie one day, she held up a £5 note and said: "This is what I got off Mac for my journey!"
Asked by McKenzie's lawyer if she considered misconduct had transpired, she said: "A nod's as good as a wink to a blind horse!"
McKenzie was worth at least £30,000 (the equivalent of about $3.1 million today), the Herald said.
Bert Brooks sought damages of £4000. However, he accepted McKenzie's offer of £1250 (about $129,000 today) which was made without admitting to the evidence given in the case.
The jury found Cecilia Brooks had committed adultery, and awarded the offered damages of £1250.
The Chief Justice, Sir Robert Stout, said the money would not go to Bert Brooks, but to the benefit of his children and possibly Cecilia Brooks.