A former Auckland nun watched Muslim militants use an axe to cut off the right hand of her younger brother in India, but says she has forgiven the men.

"We forgive everyone," Sister Marie Stella Thenganakunnel, 59, who was based at two Auckland schools last year before going to teach in the Cook Islands, told the Asia News newsagency. "We bear no grudge or resentment."

Originally from South India, Sister Marie Stella spent time at St Mary's College, Ponsonby, and Sacred Heart College, to become familiar with the New Zealand educational curriculum, then moved to Rarotonga to teach at Nukutere College.

She went back to Kerala state in India's south on holiday in April, but her brother, Professor T J Joseph, 53, asked her to stay on. He had triggered a religious row by giving exam candidates a punctuation exercise framed as a dialogue between God and the Prophet Mohammed. The controversy led to Muslim protests, and his suspension as head of the language department at Newman College in Thodupuzha, and he was arrested by police in April and released on bail.

On Sunday morning a "gang of anti-social elements" attacked Prof Joseph, his wife, Sister Marie Stella, and their 81-year-old mother as they drove home from an early mass, The Hindu newspaper reported.

Six men pulled the teacher from his car, laid him on the road and "then one man started to chop him like firewood", one of Sister Marie Stella's fellow nuns told her New Zealand colleagues.

His hand was cut off and thrown away, and other limbs were cut while the attackers ignited firecrackers to keep passers-by at bay. Prof Joseph's mother and his wife both fainted and the nun is apparently the only eyewitness, though the attackers also tried to choke her.

The Press Trust of India newsagency reported Sister Marie Stella carried her brother's hand in a polythene bag to a hospital in Kochi, 50km away, where surgeons spent 16 hours in multiple operations stitching it back onto his arm.

She told journalists that right after the attack many Muslims expressed solidarity with her brother.

"We live in a Muslim environment," she said. "Muslims are good people. Many have donated blood for him. Unfortunately, a small fringe carried out this attack. My brother, however, has only talked about forgiveness, forgiveness and forgiveness."

Her brother was "a martyr for Islamic-Christian dialogue, in Kerala and around the world", she said.

She said he had loved his work, and prepared lessons to teach his students humanist values, encouraging rational thinking, objective assessment. He also wrote on non-violence.

Two men have so far been arrested, but police have also received death threats.

Sister Elizabeth Browne-Russell, who is with the Sisters of Saint Joseph of Cluny order in Auckland, said Sister Marie Stella spent seven months in Auckland but now found herself in a "very very horrific and sad situation".