A former debt collector has been arrested by police investigating the Angela Blackmoore murder cold case and believe he arranged her killing.

A 47-year-old from Wanaka is the third person to be charged over one of New Zealand's longest cold murder cases.

He has been charged with murder and will appear at Queenstown District Court tomorrow.

Detective Sergeant Todd Hamilton said in a statement this afternoon: "We continue to support Angela's family and loved ones through this process. As this remains before the courts, we are not in a position to make any further comment."

Blackmoore, a 21-year-old mum who was 10 weeks' pregnant, was stabbed 39 times in her house on Vancouver Cres in the Christchurch suburb of Wainoni on the blustery, warm Thursday evening of August 17, 1995.

Her 2-year-old son Dillon was sleeping in his bedroom at the time of her death.

Her partner, Laurie Anderson discovered her body when he returned home after work at 11.20pm that night.

But last October there was a major police breakthrough, with the arrest of a 45-year-old man and 47-year-old woman.

At the High Court in Christchurch in February, Jeremy Powell, 45, pleaded guilty to beating Blackmoore to death.

The summary of facts says Powell turned up to the victim's house in Christchurch with a bat, and a large knife hidden in his trench coat.

Powell then admitted beating her over the head with the bat - until she was dead.

Blackmoore suffered 39 wounds to the head and neck.

Powell claims he was to be paid $10,000 for killing her.

He is yet to be sentenced.

The 47-year-old woman, who has interim name suppression, denies any involvement and her case is still going through the courts.

Time has done little to ease Anderson's pain and suffering at losing the woman he loved.

"It's virtually the same as when it happened," he told the Herald last year.

"It's still raw, you just learn to live with it. It's like a disability – say you lost your arm – you learn to live with your loss."