The parents of a teenage girl who police believe was bullied into taking her own life have made a plea for justice.

Hayley-Ann Fenton, 15, lay on her lounge floor convulsing when she got a text message: "Go kill yourself, I don't care".

It was from her "first love", a 27-year-old man she had met working at McDonald's.

He had stood by and watched as his wife typed the message, a reply to Hayley's call for help: "I don't want to die now come and get me".

The wife had been intercepting Hayley's suicidal text messages all day, replying with abuse.

A police summary of facts to the District Court in Rotorua says Hayley tried to take her own life.

Twelve hours after the dying teen was rushed to hospital, her father had to turn off her life support.

Hayley's parents, Kevin and Lesley Fenton, said the courts had failed to hold anyone accountable for their daughter's death.

The man, Pelesasa Tiumalu, was last week jailed four years and three months for having sex with a minor, and his wife, Elina, was earlier given a nine-month suspended sentence for intimidation and threatening to kill.

"How is that fair? Why were the charges threatening to kill and intimidation?" Mrs Fenton asked. "Our daughter's life was taken."

Sensible Sentencing Trust national spokesman Garth McVicar said bullying was becoming more common and there had to be consequences.

"This would certainly not be the first death by suicide as a result of bullying," Mr McVicar said.

"There should have been charges laid against it because that's the only deterrent, and it could've brought some sort of closure to the family."

If the current law did not hold people to account, there had to be a change, he said.

"I don't know of a more despicable crime than one that causes a parent to lose their child. In my opinion, there's no doubt the young girl would've been alive today if not for the abusive nature of the texts she received."

Crown prosecutor Amanda Gordon said police had carefully considered other charges but felt there was not enough evidence to substantiate them.

Auckland University criminal law professor Warren Brookbanks said police would have difficulty proving murderous intent or that manslaughter was caused by an unlawful act that was serious enough.

"Bullying is a nasty thing and people should be punished for it, but it's another thing to go on and say, 'Your intimidation caused a person's death and you should be convicted on a much more serious crime'."

A jury was unlikely to be convinced that intimidation justified a manslaughter charge, Dr Brookbanks said.

Intimidation was not on the same level of seriousness as, for example, dangerous driving, he said.

Elina Tiumalu, now pregnant with Tiumalu's child, said outside court last week that she was sorry for threatening Hayley. "I do have regrets. I feel sad but at the end of the day I'm only human."

After Hayley's death, Mr Fenton, a harvester driver, worked seven days a week and drank heavily for a year to distract himself.

He says he snapped out of it on July 18, the anniversary of Hayley's death - "for her memory", he said.

The incident happened days after Tiumalu's estranged wife returned and he broke off his relationship with Hayley.

"[Tiumalu] could've told us he was splitting up with Hayley, and she would've been alive today."