The scene of a fatal crash in which Laura Jessop, 19, died. Both Jessop and Eamonn Woods were thrown from the car and Woods later denied being the driver.

Defendants are getting off driving charges involving death or injury – simply by denying they were the driver.

Among those to use the defence was a Wairarapa man whose 19-year-old girlfriend, Laura Jessop, died in a fatal smash.

Auckland lawyer Stuart Blake says the use of the "identity defence" is widespread and increasing, and his first instruction to a client involved in a fatal crash is always: don't say whether you were at the wheel.

"In court, I'll never concede my client was the driver because one, they're entitled to remain silent and the onus is on the police to prove everything, and two, if [police] can't prove they're the driver, my client deserves the benefit of that," he said.

Fellow Auckland barrister Gary Gotlieb said there was a possible knock-on or copy-cat effect when a high-profile defendant was not convicted because it could not be proved they were driving, leading others to try raise the defence.

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Identity-related dangerous driving cases that have hit the headlines in recent years include those that never even reach court - sometimes for tragic reasons, such as the deaths of three Kiwis in Australia last week.

Southlander Kadin Wallace, 24, is understood to have committed suicide on the night of February 19 after the ute he was driving rolled south of Perth, killing his friends Dylan Sherriff and Jeremy Vergeer.

In other cases the identity defence fails, such as when Tararua teen Norman Harris tried to dodge blame for killing his 16-year-old passenger Cheyenne Ihaka in a head-on crash near Eketahuna in 2014.

Harris moved his friend's body into the driver's seat, then told police he had been asleep in the back.

But his ruse was uncovered, and he is serving home detention.

Whether you were the driver or not, stating that you weren't may mean the issue is never put to the test.

In Masterton, Eamonn Woods was acquitted in September of killing girlfriend Laura Jessop, 19, in a single-car crash near the Wairarapa town in 2014.

Both were thrown from the car in the accident, to which there were no witnesses.

Woods claimed Jessop was driving, and a judge dismissed the charge, saying police had not conclusively ruled out that possibility.

Laura Jessop's mother Carolyn Jessop said Woods' defence was his business.

"Whether he was the driver or not, he has to live with losing a mate."

And she says she's accepted she'll never know who was behind the wheel.

"The only people who know that are Laura and Eamonn."

In Auckland, property developer Jonathan King obtained a permanent stay of proceedings after he was charged with dangerous driving and driving drunk following a 2012 crash.

Asked about the charges in January, King's lawyer Alistair Haskett said the charges were "mere allegations."

"There was never a likelihood of police establishing any of the allegations against Mr King, including the allegation that he was driving or that he was over the legal alcohol limit."

Model and socialite Sophia Nash was the other occupant in the car - she was not charged.

King's lawyer Alistair Haskett wouldn't discuss the specifics of the case, but said he often found that even if an admission had been made, police may not have followed proper procedure.

"Even where you get a driver who has admitted liability or has admitted driving, often you're in a position to challenge the admissibility of the statement."

"It's a question... of whether the individual officer has taken shortcuts, or been a bit gung-ho in terms of how they approach an investigation."

Blake said one reason the defence works is that police prosecutors and investigators lack resources to prove beyond reasonable doubt who was driving.

"I do have a bit of sympathy for police because their resources are so stretched - I can tell by the quality of the preparation of their cases."

The Police Prosecutions Services' Inspector Steve King says how those charged choose to defend their case is a matter for them and their legal counsel.

"Police, through the Crown, presents the best evidence available, and it is ultimately for the court to decide the outcome based on this."

Police claim in charges involving an accident causing death, a "successful prosecution outcome" is secured more than 80 percent of the time.

But Masterton police prosecutor Jodie Lawrence - who was not involved in the Woods case - said she had definitely seen a general increase in people not admitting guilt.

"The value of life isn't what it used to be...they'll put prosecutors to the test, because they know how to play the system."

Police Association president Greg O'Connor was not aware it was an increasing problem, but added the "variability" of judges sometimes made successful prosecutions more difficult.

"It's the same problem with fleeing drivers. Everyone says why don't you get them afterwards? Well, very difficult to prove it, and who's got the time, basically."

* A previous version of this article wrongly attributed the introductory paragraph to a defence lawyer.