An Auckland woman who had an affair with a Dunedin businessman and later harassed his wife has challenged her convictions for doing so.

In February, Margaret Denise Kronfeld, 52, went to trial in the Dunedin District Court, where Judge Michael Crosbie found her guilty of criminal harassment and three charges of accessing a computer system without authority.

She pleaded guilty to two Telecommunications Act counts during the trial.

Kronfeld was later sentenced to 120 hours' community work.

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Yesterday the case came before the High Court at Dunedin, where the defendant challenged the judge's ruling.

Judge Crosbie said Kronfeld's conduct was ''nothing short of disgraceful''.

But counsel Andrew Speed - appearing by audiovisual link from Auckland - said the judge's decision to convict his client was flawed.

The court heard at trial how the affair with the married man - whose identity was suppressed - began shortly after they met at an Auckland conference in 2011.

Three years later, Kronfeld issued an ultimatum, telling the man to choose between her and his wife.

When the man chose the latter, his wife started receiving frequent emails from a variety of anonymous accounts.

Over several months Kronfeld messaged the wife, calling her ''spineless'' and telling her she was ''married to a man with a hungry, wandering penis''.

In early 2015, nearly a year after the affair ended, the defendant also accessed the married couple's email account as well as their daughter's school database.

Speed said the harassment charge was essentially based on five emails sent on April 5, 2015.

However, all but one of those emails was used to support other charges which had either been dismissed or admitted.

Crown prosecutor Robin Bates said Judge Crosbie had been entitled to look at the entirety of the evidence to provide context, but Mr Speed disagreed, saying it raised issues of ''double jeopardy''.

''Somehow you can use evidence from one charge, which is out of the picture, to support another charge runs contrary to every single principle of criminal law,'' he said.

Justice Nicholas Davison said the appeal raised a significant legal point about ''duplicity''.

Speed was also critical of the validity of convictions which stemmed from Kronfeld using passwords to access the victim's computer system.

She claimed her lover had given the password to her and she had therefore been ''authorised'' to use it.

But Justice Davison was sceptical that that authority could continue to exist following the breakdown of the illicit relationship.

He reserved his decision on the matter.

Kronfeld had also challenged her sentence but that element of the appeal was adjourned until the judge's decision on convictions was released.