Friends could have prevented the death of aspiring model Emily Longley if they had acted on a series "classic" warning signs leading up to her murder, New Zealand family violence experts say.

Briton Elliot Turner, 20, was this morning found guilty of strangling the 17-year-old New Zealander in a jealous rage at his parents' home in Bournemouth.

His father, wealthy jeweller Leigh Turner, 54, and healthcare assistant mother Anita, 51, were also found guilty of covering up the murder.

The court heard Turner had boasted to friends "I will kill her. I will go to prison and still be a millionaire when I come out" before he murdered his girlfriend.

Before the murder, he practiced strangle holds on friends so tightly they panicked and could not breathe and asked them to kill his Emily with a hammer.

Shine, a New Zealand family violence prevention agency, said Emily's death was preceded by "classic" signs of domestic homicide, including escalating obsessive, possessive, jealous, and violent behaviour.

Its services director Jill Proudfoot said it was sickening to hear how friends refused to act despite listening to Turner talking about how he could kill Emily.

"We frequently hear about this type of behaviour in the work that we do with victims who are at high risk of being killed themselves.

"This case had many clear warning signs: Turner's previous violence to Emily, his extreme jealousy, and his clear statements of intent to kill her.

"He believed his violence was justified because of Emily's actions."

"Effectively, Turner wanted the last word, and he was going to ensure this happened come what may. From asking his friend to kill Emily with a hammer, to describing a number of ways he could kill her himself, to the fact that he did not ring for help until hours after 'discovering' her dead.

"We totally support the verdict of murder. Emily's family have lost a young daughter who should still have most of her life still ahead of her. Our thoughts are with them today."

Ms Proudfoot said murderers are often men who don't 'look the type'.

Men like Turner are adept at using their good education and clean-cut appearance to hide violence and deny responsibility for the harm they cause, she said.

TURNER: 'POSSESSIVE AND VICIOUS"

Turner, a self styled 'gangster' met Emily on a double date in December 2010 and they quickly became boyfriend and girlfriend.

However, he became increasingly suspicious Emily was having affairs during their four month relationship and began fantasising about killing her.

Public school educated Turner had gone "absolutely nuts" after he found flirty messages on Emily's Facebook account and threatened to kill her with a lump hammer.

The trainee insurance underwriter also "flipped out" when he saw photos of her with "Cheeky Buff Butlers" as part of a modelling assignment.

Prosecutor Timothy Mousley QC said : "He's remorseless, he's controlling, he's possessive and he's vicious."

Turner practiced strangle holds on his friends so tightly they "began to panic and couldn't breathe."

He even told friends he had murdered Emily with a hammer on the night before her murder.

High Court judge Linda Dobbs will sentence Turner tomorrow and said he will receive a life sentence with a minimum of 15 years.

She released the parents on bail and adjourned their case for sentencing reports but warned they can expect to receive a jail sentence.

By Hayden Donnell Email Hayden, Ben Ellery of Solent News Agency