Prime Minister John Key said the Shi'ite law in Afghanistan saying a wife is obliged to fulfil the sexual desires of her husband is "unacceptable". But it would not threaten New Zealand's commitment to Afghanistan.

Mr Key said last night that he would write to Afghanistan President Hamid Karzai to express New Zealand's views on the law.

"But there is no doubt that our voice will be strongly heard, that we find this an abhorrent act and totally unacceptable to the New Zealand Government."

But Mr Key is unlikely to follow the example of Canadian Prime Minister Stephen Harper who said it would lead to "a clear diminishment in Allied support" in Afghanistan.

British Prime Minister Gordon Brown has telephoned President Karzai about the issue.

"I think in the short term, it would be unlikely to have any impact on our commitment to Afghanistan," Mr Key said. "We are fundamentally there to try and reduce the threat of global terrorism. We need to deal with that situation first and foremost."

In a copy of the law obtained by Reuters, Article 132 states "a wife is obliged to fulfil the sexual desires of her husband". It also states that a husband should spend one night in every four with his wife, have sexual contact with her at least once every four months and that a woman has to wear make-up if her husband demands it.

President Karzai was reported over the weekend as saying expressions of anger over the law were inappropriate.

"Those concerns may be out of an inappropriate or not so good translation of the law or a misinterpretation of the law," he told reporters.

The Justice Minister would be studying it.

New Zealand has about 130 Defence Force personnel in the provincial reconstruction team in Bamyan and the Government is conducting a review of its efforts in Afghanistan.