SYDNEY - Judging by the success rate of some Australian contract killers, the hirers would be justified in demanding a refund.

The typical Australian contract hit costs A$16,500 ($18,400) and is initiated by a disaffected spouse - but they are often not successful, according to the Australian Institute of Criminology.

Of 28 intended victims during 1989-2002, just three were unlucky enough to end up dead, a study found.

But of 15 hits associated with organised crime, there were 15 deaths, with just two offenders arrested.

Contract killings made up just 2 per cent of all homicides over the past 13 years in Australia.

The study's authors, institute researcher Jenny Mouzos and South Australian police Detective Inspector John Venditto, examined 163 cases - 69 where the victim was killed (34 unsolved cases) and 94 where the attempt failed.

The most common motivation was relationship breakdown, where the services of a contract killer were sought by a current or former lover. The aims included eliminating a partner to be with a new lover and gaining custody of children. Males (63 per cent) were more likely to be the instigators.

Ms Mouzos and Mr Venditto suggested the organised crime-related contract killings succeeded because of better access to professional hitmen.

That avoids the pitfalls that apply to others - having to put the word out among friends, associates and strangers to find someone willing to commit murder for money. Inevitably, many of those approached went straight to police.

The study noted three classes of hitmen - amateurs, semi-professional and professional.

Amateurs were impulsive and disorganised and were frequently caught.

In contrast, professionals planned the hit and left little evidence behind.

The average payment for a contract was about A$16,500, ranging from a low of A$500 up to A$100,000 offered to a hitman for two contracts.

Meanwhile, it appears the professionals may not always make their target. New South Wales police are investigating the execution-style slaying of Sydney man Mark Nicholls, who was gunned down in a city panelbeaters last Sunday.

It is thought Nicholls was shot by mistake.