Foreign Affairs officials are warning the Government that its hardline sentencing and non-parole policy risk damaging New Zealand's international reputation.
They say National's "no parole for the worst murderers" policy and the proposed "three strikes and you're out" law could breach international obligations on torture and civil rights.
The Ministry of Foreign Affairs and Trade says such breaches would affect New Zealand's ability to influence other countries.
The ministry's advice, obtained by the Herald under the Official Information Act, says passing the laws "would pose reputational risks to New Zealand by resulting in international criticism".
The ministry has told the Government that no parole for the worst murderers - a National election policy - would enable "indefinite detention without the possibility of release", and would probably violate two human rights conventions monitored by the United Nations.
Act's "three strikes" policy, which 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 National's Sentencing and Parole Reform Bill.
National has apparently not heeded the advice, which was given 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.
The Foreign Affairs officials said the measures would most likely violate the UN-monitored International Covenant on Civil and Political Rights and the Convention against Torture and other Cruel Inhuman or Degrading Treatment or Punishment.
The result could be "increased communications" against New Zealand.
"These bodies may formally find New Zealand in breach of its international obligations. A breach of New Zealand's obligations is a serious matter due to the general obligation at international law that treaty commitments must be complied with in good faith."
The ministry said New Zealand had to report to the United Nations on its compliance with international treaties, and any relevant changes to legislation were reported as part of that process.
National intends to abolish parole for repeat violent offenders, meaning a life sentence will mean murderers with previous convictions would never be released.
The policy would also give judges the option of sentencing those responsible for the "worst cases of murder" to life with no chance of release even if they didn't have a record.
The ministry's advice on three strikes cited the example of an offender whose third strike was a robbery conviction that would normally earn a sentence of no more than 10 years. Instead, the offender would receive a life term with a 25-year non-parole period, which was "disproportionate".
National introduced three strikes to Parliament as part of its support agreement with Act, but has reserved judgment on supporting it any further.
Attorney-General Chris Finlayson has warned his National colleagues that "disproportionate" sentences under it may breach New Zealand's Bill of Rights.
David Garrett, the Act MP who designed "three strikes", said he wanted to know what countries the Foreign Affairs officials believed would be offended.
"They shoot people in China for much less and we have just concluded a free trade agreement with them. And we can't be offside with the Yanks because half their states have three strikes."