The end of a three-year affair with a Dunedin businessman allegedly led an Auckland woman to harass her former lover's wife.

Margaret Denise Kronfeld, 51, is accused of being behind various anonymous messages and phone calls to the complainant and also accessing a web-based "parent portal" of St Hilda's Collegiate where the couple's daughter went to school.

The trial before Judge Michael Crosbie without a jury began before the Dunedin District Court yesterday.

Kronfeld is accused of three counts of accessing a computer system without authority, three of unlawfully using a phone, using offensive language and criminal harassment in the wake of her affair with the married man ending.

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The court heard how the successful businessman had met the defendant at a "leadership workshop" in Auckland in May 2010.

He was the first to give evidence yesterday and sheepishly spoke about the origins of the relationship.

"I take it this isn't the first affair you've had," defence counsel Andrew Speed began in cross-examination.

The man denied he had been unfaithful before but admitted he had to be split up from Kronfeld by the organiser of the workshop they were attending because of their inattention.

Speed said his client received text messages from the man making his intentions clear.

"Those were flirtatious texts to keep her interested and make her think you were witty and charming," the lawyer said.

The witness accepted they "seemed funny at the time".

Speed also accused him of attending a strip club and meeting prostitutes on the Auckland trip.

That was vehemently denied but the man said a Central Otago-based colleague had indulged in those activities.

The affair involved trysts around the country and the pair were spotted by someone who knew the man while dining in Ponsonby.

"Best you stick to your home-made lunches," the associate texted him later.

In February 2014, after more than 50 secret meetings, the married man took the advice and ended the affair.

"I had to make a decision about staying with my wife or being with Marg. It wasn't fair to either Marg or [my wife] and to my family and so I made the decision to end it and stay with [my wife]," he said.

Kronfeld was unhappy but did not seem "overly upset" during the phone call, the witness said.

Within weeks, the man's wife was inundated with emails from a variety of identities and there was more.

"I started to get a whole lot of unknown calls with nobody speaking," she told the court.

The complainant, who has permanent name suppression, said one of the emails called her a "weak bitch" while others said she was "worthless" and one suggested she find herself a lover - "tit for tat".

Kronfeld allegedly wrote the woman was "married to a man with a hungry, wandering penis".

There was also one addressed to the couple's daughter, calling her mother a "failure" and father an "adulterous man" and asking them to "fill in the gaps".

Kronfeld was copied in to some of the messages, the court heard, as was the complainant's manager

"It feels just like constantly being taunted," the woman said from the witness box.

"It's like you're in some nightmare."

Her husband said he had used an Xtra account to communicate with the defendant during their affair and became concerned someone other than him was using it after he had broken things off.

In the inbox were instructions on how to access the St Hilda's Collegiate parent portal, which Kronfeld is accused of doing.

The trial is scheduled to last three days.