A British sailor charged on Anzac Day with having excess breath alcohol twice the drink-driving limit escaped prosecution after high level intervention.

Chief petty officer Lauren Cobby, 29, who was in New Zealand on an exchange visit, was charged by police after she crashed a vehicle into a parked car along Onewa Rd, on the North Shore.

Her breath alcohol reading was 829mcg. The legal limit is 400mcg. She had been drinking with other naval ratings at the Birkenhead RSA.

A spokesman for New Zealand Defence Force said yesterday its lawyers contacted police to raise the legal issue of the Visiting Forces Act. This allows the courts to dismiss charges for overseas service personnel.

Official Information Act documents released to the Herald on Sunday show Cobby's case went to the Chief of Defence Force, Lieutenant General Jerry Mateparae, and Defence Minister Wayne Mapp.

The British High Commission and the Attorney General's Office were also informed.

Mapp's spokesman said he could not comment because he was overseas.

Alcohol Healthwatch director Rebecca Williams said letting Cobby off the offence was a poor decision.

"There have always been people who appear to be treated differently. It sends a bad message to people," said Williams.

"Drink-driving can be deadly and the penalties of that should apply to everybody. When people come from overseas it should be expected that they abide by the same rules."

Cobby first appeared at the North Shore District Court in May. She was discharged without conviction in June. She left New Zealand in July.

Police took into consideration Cobby paid the full cost of the damage to the rental car she was driving and the vehicle she struck, the Herald on Sunday understands.

The Visiting Forces Act 2004 - which is applicable to Ex Longlook exchange personnel - states that criminal proceedings may be launched in a New Zealand court only with the consent of the Attorney General.

Auckland criminal defence lawyer Karen Harding said a discharge for drink driving was rare.

"She would have established grounds under section 106 of the Sentencing Act, which says that the consequences of a conviction would be out of proportion to the circumstances of offending," said Harding.

"Often, it's because it would ruin their career. Someone in the Navy could not be granted entry to other countries, for example."

Birkenhead RSA vice-president George Coffin said he spoke to Cobby about half an hour before she drove on Anzac Day.

"I did not think she was over the limit. If I did I would have told her to take a taxi," he said.