Two women agreed to fight over a man; one brought a knife.

Tayla Annette Blewden (18) was enraged when she found out the fellow she believed was her boyfriend might be engaged to someone else.

So she phoned the love rival and challenged her to a fight.

December 11, 1.30pm, in Brockville Rd, Blewden and an associate waited at the gate of her home.

The victim, with her handler, turned up at the appointed time.

But it was not to be the fist fight she had anticipated.

''Queensberry rules [laws of boxing] were not observed,'' Judge John Macdonald said.

Blewden whipped out a knife she had taken from her kitchen and charged at her opponent.

''In a sweeping motion she swiped the knife at the victim's upper body,'' court documents stated.

The blade scratched the victim's neck, resulting in a superficial 6cm scrape.

The victim lashed out at Blewden and there was brief verbal fracas before the parties went their separate ways.

Blewden pleaded guilty to assault with a weapon and appeared in Dunedin District Court this week.

Judge Macdonald said it was unusual in a case of violence that the victim had essentially consented to the brawl.

Prosecutor Sergeant Chris George stressed the ''massive potential for serious injury'' and the judge said he hoped the defendant appreciated it was simply good fortune the victim's wound was not more severe.

The love triangle, he said, had prompted an understandably enraged response.

But he questioned Blewden's choice of target.

''I'm unsure why you took it out on this victim rather than your so-called boyfriend,'' Judge Macdonald said.

A report before the court noted Blewden's propensity to act impulsively ''even irrationally'', but assessed her as a low risk of reoffending.

Defence counsel Alex Bligh said her client's mental health had improved and she was currently flourishing on a hairdressing course.

Blewden was sentenced to six months' supervision and five months' community detention.

The judge refused to make a reparation order in favour of the victim, since she had been somewhat willing to participate in the scuffle.