Acting Prime Minister Winston Peters says he'll personally contact the Governor of California if MP Nikki Kaye's step-brother is sentenced to death.
Clinton Thinn has been convicted of first-degree murder for killing a fellow inmate in a Californian jail, and he's facing the death penalty.
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"This is a really tough situation, and obviously I'm in touch with my family," Ms Kaye told Newshub.
The Auckland Central MP says she's conscious of separating her political role from her role as a step-sister.
"There could be legal considerations, and anything that I say could affect that."
California has the highest number of death row inmates in the United States, although no one's been executed since 2006.
If Thinn is sentenced to death, the New Zealand Government would rally, and the Acting Prime Minister says he'd personally contact the state's Governor.
"As a Foreign Minister I'd do that, yes, and with the full support of the total Cabinet," he told Newshub.
Thinn was in jail for an attempted bank robbery when he strangled a fellow prisoner with a shirt.
In 2014 Kiwi Peter Gardner was jailed in China, facing the death penalty for smuggling methamphetamine. In 1982 Lorraine and Aaron Cohen were sentenced to death in Malaysia for heroin smuggling.
"Regardless of which country it was and regardless of whether it's the United States, we always made it very clear that we're opposed to the death penalty," says Mr Peters.
Thinn may avoid the death penalty based on special guidelines particular to California.
A murderer can only be sentenced to death in special circumstances, such as murdering a police officer, a witness or a judge, or murdering because of race or religion, or if the murder involved torture or poisoning.
Annaliese Johnston from Amnesty International says it condemns the possibility of Thinn's execution.
"We believe it's the ultimate cruel, inhumane and degrading treatment."
New Zealand only abolished the death penalty in 1989, when it was still in place for treason. Since then we've joined a global community of countries opposing capital punishment.