Foreign Affairs officials fear New Zealand's international reputation could be damaged by the Government's hardline sentencing and non-parole measures.
The officials have warned National its "no parole for the worst murderers" policy and the proposed "three strikes and you're out" law could breach international obligations about torture and civil rights to which New Zealand is a signatory.
The Ministry of Foreign Affairs and Trade says such breaches "carry a moral and political force and impact on New Zealand's ability to effectively influence other states".
The ministry's advice, obtained under the Official Information Act, says the enactment of the laws "would pose reputational risks to New Zealand by resulting in international criticism".
The ministry has advised the Government that no parole for the worst murderers - a National election policy - would allow for "indefinite detention without the possibility of release", and would likely violate two major human rights conventions monitored by the United Nations.
The ministry said "three strikes" - the Act New Zealand policy that imposes a life sentence with a minimum non-parole period of 25 years on the third "strike" offence - "may result in disproportionate sentences that could also breach the human rights obligations assumed by New Zealand [and most other countries]".
"[This is] potentially in violation of New Zealand's obligations not to arbitrarily deprive individuals of their liberty and not to employ cruel, inhuman or degrading treatment," the ministry said.
The two measures are before Parliament in the Sentencing and Parole Reform Bill.
National has so far ignored the advice, which was given to the Ministry of Justice to go to Cabinet for consideration two weeks before the Bill was introduced in February.
Labour leader Phil Goff said the officials' fears were "another downside" to the measures which were "political rhetoric" of questionable effectiveness.
Justice Minister Simon Power said he expected the issues to be debated at the Bill's select committee stage, which he would follow with interest.
The Foreign Affairs officials said the measures would most likely violate the International Covenant on Civil and Political Rights and the Convention against Torture and other Cruel Inhuman or Degrading Treatment or Punishment.
Both conventions are UN-monitored, and the ministry warned the measures could result in "increased communications" against New Zealand.
National intends to abolish parole for repeat violent offenders, meaning a life sentence will literally mean life for murderers with a record.
The policy also gives judges the option of sentencing those responsible for the "worst cases of murder" to life with no chance of release if they did not have a record.
