Police have charged a US citizen with assisting suicide but cannot have her extradited from her home in North Carolina.

Detective Sergeant Scott Armstrong said they believe the 48 year-old woman helped Auckland woman Audrey Wallis commit suicide in 2007.

Mr Scott said police have been helped by Interpol, the US Department of Justice and the FBI.

He said assisted suicide is a charge under the Crimes Act and carries a maximum penalty of 14 years imprisonment.

However, the charge is not covered by the Extradition Treaty between New Zealand and the US.

"A number of charges that do fall within the Treaty were considered but they fell below the evidential threshold," Mr Scott said.

He said the woman has refused to return to New Zealand voluntarily.

"The warrant will remain active should the defendant choose to return to New Zealand at any stage in the future," Mr Scott said.