The man who killed Sara Niethe has already had two stints in prison for violence and drug dealing since her death nearly 10 years ago.
Mark Edward Pakenham was originally charged with the murder of the Kerepehi woman but on Friday pleaded guilty to the lesser offence of manslaughter, after a review of the evidence by the Crown.
Ms Niethe's body is still missing.
Pakenham was arrested in August 2011 after a renewed investigation by Waikato police which discovered that he had made partial confessions to associates that he was implicated in the cold case.
The 51-year-old will be sentenced in June for manslaughter, committed by unlawfully administering methamphetamine, which carries a maximum penalty of 10 years in prison.
He also admitted two counts of injuring with intent against two women in the early 1990s.
But the Herald can reveal Pakenham has a lengthy criminal record dating back to 1978 which includes assault, threatening to kill and possession of a large amount of methamphetamine.
In June 2008, he was sentenced to two years and three months after police found an ounce of P in his car with a street value of $28,000, as well as small zip-lock bags and a set of electronic scales.
Pakenham pleaded guilty to possession of the class-A drug for supply, but his lawyer argued that the ounce - a 28-gram amount typically used by wholesale suppliers - was for personal use.
The sentencing notes of Justice Edwin Wylie reveal Pakenham had a "happy childhood" growing up in Auckland until he was expelled from Howick College as a 14-year-old.
He married at 23, had two children, then another two children in a different relationship.
The probation report said he did not know where his former partner and children lived, and had no contact with his own parents and wider family.
His brother is Brett Pakenham, a respected Auckland detective with expertise as a criminal profiler who has worked overseas tracking war criminals.
Mark Pakenham has been addicted to methamphetamine-type drugs since he was 30, according to the judgment, and admitted "falling off the wagon" after attending rehabilitation programmes.
The addiction and his criminal associates led him to sell drugs to finance his own habit, Pakenham said.
His criminal history dates back to 1978 and includes a prison sentence in 2005 for assault, breach of protection orders, threatening to kill and wilful damage.
Of the 2008 methamphetamine conviction, Justice Wylie said it was clear Pakenham had a drug problem which he intended to address.
"I trust you will make good on that intention," said Justice Wylie. "If not, the consequences for you are likely to be very serious in the future."
Mark Edward Pakenham
* Aged 51.
* Pleaded guilty to manslaughter of Sara Niethe and injuring two other women in the early 1990s.
* Has a long criminal record including two stints in jail since Ms Niethe disappeared in 2003.