A prosecution against an alleged tomahawk-wielding robber will likely fail after police unlawfully obtained his DNA.

The Court of Appeal has today dismissed a Crown appeal and commended a District Court judge's ruling that saw Ian Toki's DNA illegally taken and stored in the police databank.

The decision also criticised the police reliance on its databank, which in 2011 seemingly had samples acquired using incomplete forms that omitted several statutory rights.

In August 2011, when Toki was 17, he was interviewed by Constable Judith van de Lande at the Kerikeri Police Station after being charged with stealing a car, court documents show.

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Van de Lande requested Toki give a bodily sample and provided him with a copy of the police template for giving DNA.

He initialled each page, but, unknown to Toki, the officer had given him an illegal form used by police at the time which for "unexplained reasons" omitted several rights under the Criminal Investigations Act.

One of the requirements left off was the ability for someone to withdraw consent to police use of the sample.

Toki, who was under no legal obligation to give a sample and was also afforded the right to seek counsel beforehand, took a mouth swab under the constable's supervision.

Van de Lande did advise Toki verbally that his sample would be stored on the databank and could be used by police to investigate future crimes.

But she failed to inform him about how he might withdraw his sample from the databank.

There is a five-month window for which he could have exercised that right.

Toki was convicted on February 2012 of the burglary, and under law the conviction extinguished his right to withdraw his consent to the sample.

Toki was further convicted of eight more offences - for driving while disqualified and for breaches of community work and conditions of supervision.

Then in April 2015 he was charged after two men allegedly forced entry into a Christchurch home with a tomahawk.

"The principal offender assaulted three of the occupants with varying degrees of severity. Apart from punching them, he struck one in the head with a frying pan," the Court of Appeal decision reads.

Witnesses also identified the alleged lead attacker as drinking a bottle of beer which was left on the home's doorstep.

DNA from the bottle matched that of Toki's in the databank, police found.

During a pre-trial application last year in the Christchurch District Court, the Crown sought to introduce Toki's DNA as evidence. But Toki objected, arguing he did not give his informed consent and only signed under van de Lande's direction.

When determining the Crown's pre-trial application, Judge Brian Callaghan rejected Toki's claim but did find he was not advised of the specifics of withdrawing his sample, nor the process to effect the withdrawal.

"I have found the breach of the defendant's right is a significant one," Judge Callaghan said.

"The police were careless to a high degree. The legislation is specific as to what a defendant must be told and given," he said.

Judge Brian Callaghan said it was "not an attractive option for the defendant to escape conviction on serious charges" because the evidence is inadmissible.

"But if he is to be convicted, which he may well be, there will always be a question that his self-incrimination arose from an unlawfulness," he added.

Following the Court of Appeal hearing on October 25, Justice Stephen Kos, Justice Rhys Harrison and Justice Murray Gilbert endorsed Judge Callaghan's finding.

The Crown argued Toki's DNA was the only route in its case to identifying him as the alleged offender in the tomahawk robbery.

But the Court of Appeal judges said police "elected without explanation to adopt a defective procedure".

"They must accept the consequences ... The sampling process constituted gross carelessness," the judges said.

They said the prosecution against Toki will "likely fail" without the DNA sample.

"The seriousness of the violation to Mr Toki's person does not consist solely in a discrete instance of physical contact. It extends to denying him the opportunity to consent properly to a procedure which enables the state to conduct ongoing surveillance of his behaviour with molecular precision," the judges said.

"The enduring ability for the state's law enforcement agencies to identify Mr Toki's presence at a particular place raises issues about self-incrimination if the initial bodily sample was tainted by lack of consent.

"Moreover, it is trite that DNA is not a mere fingerprint: it contains a wealth of genetic information about a person with unlimited future utility."

The judges said the one-off intrusion "permanently erodes Mr Toki's privacy and freedom" which would usually be beyond the state's reach.

"An effective and credible system of justice will not tolerate lightly the reliance of the police on the DNA profile databank if the underlying sample was taken contrary to clear legislative prescriptions and in abrogation of the person's rights," they concluded.