A rights group urged New Zealand Prime Minister Jacinda Ardern to raise concerns about reported Chinese government abuses of Muslims during her visit to Beijing next week.

Ardern's visit on Monday and Tuesday comes two weeks after a gunman killed 50 worshippers at two mosques in New Zealand.

Human Rights Watch said Friday that Ardern "spoke forcefully in defence of Muslims' rights" after the attacks and should do so again in Beijing.

The prime minister is reducing the length and scope of her China visit in response to the mosque attacks.

Hundreds of thousands of Chinese Muslims have allegedly been detained without charge in internment camps, where relatives and ex-inmates claim they have been subjected to physical abuse and forced to renounce Islam.

Human Rights Watch said Ardern should publicly call on Chinese leaders to close the camps, end the abuses, and permit independent, international observers' access to the Xinjiang region that is home to China's largest mainly Muslim minority group, the Uighurs.

Other members of Muslim groups, including Kazakhs and the Hui have also allegedly faced internment and other forms of repression.

China has so far rejected calls for independent observers and, after first denying the camps' existence, now says they are voluntary vocational training centres that provide job skills to ward off religious extremism.

Although Muslim countries have remained largely silent over the camps and other abuses against Chinese Muslims, some European nations and the US have become increasingly vocal in their criticism.